What is an isotope? a. a form of an element with a different number of electrons c. a form of an atom with a different amount of orbitals b. a form of an atom with a different number of protons d. a form of an atom with a different number of neutrons

An isotope is an element with different atomic mass.

We have,
Atomic mass = no. of protons + no. of neutrons 
Now, we know, protons never changeso the only thing that changes is neutrons if we want different atomic mass.

S,

The answer is "D. A form of an atom with a different number of neutrons"

The answer is not A. because same element with different number of electron is ion.

The answer is not B. because if it has different number of protonit is going to be a different element. Just like our fingerprints, we can say that protons are fingerprints of element. No. of protons never change.

The answer is not C. because again talking about electrons.