The answer is A , an enzyme.
An enzyme is defined as a molecule which is protein in nature, that helps other organic molecules enter into chemical reactions with one another but is itself not affected by these reactions. It remains the same as it was before and after the reaction. In other words it acts as a catalyst for organic biochemical reactions.
Enzymes are usually very selective in the molecules that they act upon, called substrates, often reacting only with a single substrate.
Enzymes can quicken reactions by a thousand fold but will only be able to work within a narrow range of temperature and pH, outside of which they can lose their structure and become denatured.
