Answer:
The correct option is A. Substrate
Step-by-step explanation:
Enzymes can simply be described as proteins which are used as catalysts i.e they speed up a reaction. The amount of enzyme does not increase or decrease in a reaction, though the shape of the enzyme might change in a reaction.
Substrate can be described as a substance or molecule that fits into the active site of the enzyme. The active site of the enzymes are specific for a particular kind of substrate.