Answer:
B. Succinate is the substrate, and fumarate is the product in the reaction.
Explanation:
The substrate is the reactant, i.e. the compound that is being acted upon in the reaction. In this case, the substrate is succinate.
The enzyme is the compound that is doing the catalysis, in this case it is succinate dehydrogenase.
The product is what is made by the reaction, in this case it is fumarate
A non-competitive inhibitor is one that reduces the activity of the enzyme regardless of whether or not it has already bound the substrate. This is not true for this reaction, as the inhibitor - malonic acid - acts as a competitive inhibitor, in the presence of more succinate acid, its effects are reduced.
Therefore, the correct answer is B.