You have discovered an enzyme that can catalyze two different chemical reactions. which of the following is most likely to be correct?
a) the enzyme contains α-helices and β-pleated sheets.
b) the enzyme is subject to competitive inhibition and allosteric regulation.
c) two types of allosteric regulation occur: the binding of one molecule activates the enzyme, while the binding of a different molecule inhibits it.
d) either the enzyme has two distinct active sites or the reactants involved in the two reactions are very similar in size and shape.

The correct option is D.

Enzymes are biological catalysts that speed up the rate of biochemical reactions in living systems by lowering the activation energy required for the reactions to occur. Enzymes take part in various reactions but they are never used up during the course of the reaction.  

Enzymes are very specific in action and will only catalyze certain reactions under optimal conditions. For a single examine to catalyze two different reactions, it implies that the enzyme either have two active sites or the molecules involve in the reaction are very similar in term of size and shape.