"what part of the antibody's structure determines its class? "

Out of the following given choices;
A) light (L) chain B) constant (C) regionC) variable (V) regionD) heavy (H) chain
The answer is B. This region is called the Fc region and composed of only heavy polypeptide chains only. It is used to classify antibody according to class. There are IgM, IgA,   IgG, IgE, and IgD. Fab is the varying region of the antibody that is significant in binding antigens.

Your answer is going to be b.

Allow large amino acids through