What is an isotope? how does one isotope differ from another isotope

Isotopes are different versions of a specific element.

Every atom/element is defined by its protons. So if it has 1 proton it's hydrogen no matter what, if it has 2 protons it's helium no matter what and so on. If you know the number of protons, you know the element.

You can change the number of neutrons in the nucleus and you'll get isotopes. Atoms that have different numbers of neutrons are isotopes of each other. It's important to understand that the element is the same but one is going to be a little bit heavier or lighter than another.

a is the correct answer