Answer D. is correct. Isotopes are different forms of a single element. How this is made, if you're familiar with the structure of an atom, is that when the protons and neutrons group together, although a single element has a set number of protons, the number of neutrons is flexible. So for the element Carbon, although there is always 6 protons in the nucleus, there can be 6, 7, or 8 neutrons in the nucleus. These different number of neutrons determine the isotope number. For 6 neutrons, we call the isotope Carbon 12 (6 protons plus 6 neutrons). 7 neutrons is called isotope Carbon 13 (6 p + 7 n) and 8 neutrons is called Carbon 14 (6 p + 7 n). So, although the number of neutrons can change what the isotope is, different isotopes for the same element always have the same number of protons (answer D).
