Different types of living matter often have different forms of the same elements

Different types of living matter often have different forms of the same elements in their bodies. For example, the nitrogen in an animal often has a slightly different atomic structure than the nitrogen in a plant. Recently, nutritionists have discovered how to deduce the diets of various animal species by examining the type of nitrogen (and other elements) inside their bodies.

What is the chemical basis behind this scenario?
A) Antioxidants buffer the potential damage that free radicals do to cells.
B) Radioactive elements can be used to trace the paths of molecules through the body.
C) Hydrophobic interactions keep water molecules from forming bonds with fats and oils.
D) Isotopes have the same atomic number but different atomic masses.
E) Covalent bonds result when two atoms share electrons.