Answer:
protons
Step-by-step explanation:
Protons is the answer, because they are what determines the identity of an element. Hydrogen always has one proton. No more, no less.
You can remove electrons from an element and it is still the same element. We call it an ion when this happens: think of something like Na+ or Cl-. An electron has been taken or added to those elements, but they're still the same element!
You can remove neutrons from an element and it is still the same element. We call it an isotope when this happens: maybe you've heard of carbon-14, which has two extra neutrons. We use that in carbon-dating. It's still carbon, though!
Those other options, A B and D, are made up of quarks. This isn't the right answer, because you can have two elements with different amount of quarks because quarks make up neutrons and other subatomic particles.