This is how we can easily answer the questions:
a) Which atoms represent isotopes of each other?
To answer this question, you have to see which ones have the same number of protons, because no matter what, the number of protons does not change as well as the number of electrons in the case of isotopes. Because only number of neutrons vary.
So your answer will be atoms B and D
b) Explain why they are isotopes.
The numbers of protons and electrons remained the same whereas the number of neutrons differ.