Question

Which statement best explains how isotopes can have different masses and still be the same element?

Answer 1

Your list of choices is a very short list, and doesn't include any
correct explanation.

The mass of an atom is roughly the number of protons AND neutrons
in its nucleus, but the element only depends on how many protons are there. 

Different atoms of the same element may have different numbers of
neutrons, so their masses are different.  But they're the same element,
because they all have the same number of protons.