Should pasta be started in cold or boiling water?

As stated in another answer, Italian tradition is that all pasta is cooked in boiling water. A reasonable explanation for this usage is that it's easier to get the time right this way.

Pasta is very sensitive to cooking time, and will easily turn from 'al dente' to an overcooked mush if left on the fire a couple of minutes too much. By cooking it in boiling water, you ensure that it cooks in uniform conditions, always at the same temperature, regardless of the starting temperature of the water, the temperature of your kitchen, and the power of your burner. So it is a safer bet that cooking it for the same amount of time will work.

In my experience, the time marked on the package is almost always accurate for pasta cooked in boiling water. So it gives you a useful reference point, which you don't have if you cook it starting from cold water.

For dried pasta it doesn’t really matter if you start with cold or hot water, as most of the time pasta spends in water is for hydration. And once the hydrated starches reach a certain temperature they gelatinize, thus cooking the pasta. When you start with cold water, you should use less water, which is actually a plus...

Note: I forgot to mention, you should swirl the pot every couple of minutes to prevent sticking.

However, when you’re cooking fresh pasta, you should directly start with boiling water. As it’s already hydrated, you just need gelatinization.

As for which you should do for dry pasta, there are benefits to both:

• starting from boiling
  • more consistent timing (and less attention), since you can time from when you add the pasta
  • works with long shapes like spaghetti and fettuccine, since they'll soften quickly to bend submerge
• starting from cold
  • faster overall - less water to boil, and pasta is already starting to cook by the time it hits a full boil
  • easier to avoid initial sticking
  • starchier pasta water, useful for sauces

It depends on the pasta shape:

There are times when you do want to start with a large pot of already-boiling water. The first is when cooking fresh pasta. Because fresh pasta is made with eggs, if you don't start it in boiling water, it won't set properly, causing it to turn mushy or worse, disintegrate as it cooks.

The second exception is with long, skinny pasta shapes like spaghetti or fettucini. Because they stack together so easily, it's more likely than with other pasta shapes that they will stick together. As the pasta heats and absorbs moisture, starches on its surface gelatinize, becoming sticky, If the strands are stuck together when this happens, they'll fuse together permanently, especially in a smaller pot where you have less room to maneuver them.

https://www.seriouseats.com/2013/05/ask-the-food-lab-can-i-start-pasta-in-cold-water.html

TL;DR please be nice with yourself: only drop pasta in a pot of boiling water (approx 1 liter every 100 grams of pasta)

To the eyes of an Italian, the mere allusion to cooking pasta by dropping it into a pot of cold water is unthinkable.

You always need to drop pasta into a pot of boiling water in order to cook it. On the contrary, you'll end up eating a dish of overcooked pasta with an unpleasant gummy consistence.

The rule of thumb is approx 1 liter of water for 100 grams of pasta.

Another rule of thumb is opting for the best brands, the ones whose plants are settled in the southermost regions of Italy (Campania, Basilicata, Calabria, Puglia, Sicilia, Molise etc.).

Actually both given answers and options in the question are wrong. The highest temperature starch needs for hydration is 83°C. Water boils at 100°C, thus you don't need boiling water. Cold water is also wrong, but not because of the pasta, because of the cooking pot. The salt is made of ions which need to connect to other atoms when the salt is dissolving. If the water is cold but you heat up the cooking pot, then the ions might connect to the high energized iron atoms instead of being surrounded by low energized water molecules. Damages to the cooking pot will be visible after 10 years though. In the cooking water the salt is added to prevent starch granules in the pasta to merge with each other instead of swelling during hydration. Because if they merge the pasta lose elasticity and the sugar taste of the starch is less accessible for our tongue, making the pasta taste bad and feel clumsy. For this fact you can allegedly never add too much salt to the boiling water. Rule of thumb is 1 teaspoon for 1 liter of water. There is no rule of thumb for the ration of pasta and water, since you can even cook pasta like a risotto.

The best way to cook pasta is to heat water in a cooking pot until you see bubbles at the bottom, then add salt and then add pasta, stir occasionally. Boiling cooking water is for lazy cooks who don't want to stir all the time and cold water cooking is for soaking the pasta when you are low on heating fuel.