Zymogen or proenzyme is a : ans. Modulator Hormone Vitamin Enzyme precursor

Zymogen, also known as a proenzyme, is an enzyme precursor. This means it is an inactive substance that is converted into an enzyme when activated by another enzyme. Here is a step-by-step explanation:

1. Zymogens are produced and released by cells in an inactive form. This is a protective mechanism to prevent the enzymes from digesting the cells in which they are produced.

2. These inactive enzymes are then transported to the site where they are needed.

3. Once at the site, the zymogen is activated. This usually involves a biochemical change such as the breaking of a peptide bond within the zymogen molecule. This change alters the shape of the molecule to form the active enzyme.

4. The active enzyme can then perform its function, such as breaking down a substrate.

So, the correct answer to your question is "Enzyme precursor".