What is the main difference between an isotope and an ion?
a. an ion has extra protons.
b. an isotope has extra neutrons.
c. isotopes are radioactive while ions are not.
d. an ion always has a positive or a negative charge?

I believe the answer is D. An ion always has a positive or a negative charge. An Ion is an atom with net electric charge due to the loss or gain of one or more electrons. An isotope is each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their neutron, thus they differ in relative atomic mass but not in chemical properties. 

Ibelieve the answer is d