If there are 2 forms of C, Carbon 12 and Carbon 14, Carbon is said to have

compound

isotopes

ions

molecules

Your answer would be isotopes. You can immediately remove compound and molecule, because this would require the presence of conglomerates of carbons, not just two different, separate types. Ions can also be eliminated because they describe the charge of the carbon, not the elemental weight.

Isotopes refers to atoms that have the same elemental identity set forth by the number of protons, but have a different number of neutrons than one another. This causes the disparity in weight; both carbons have six protons, but C-12 has six neutrons while C-14 has eight.