Which statement best describes what happens to protein when a steak is cooked?
a. it denatures and coagulates during cooking.
b. it decreases in size as amino acids are lost.
c. it loses bacteria because they are denatured.
d. it undergoes polymerization to form longer chains

It denatures and coagulates during cooking.

Explanation:

Denaturation is a phenomenon in which the proteins undergo changes in their secondary, tertiary or Quaternary structures. They undergo change in their native state.  

The changes are may be due to heat treatment, application of strong acidic or basic conditions, addition of some salts or solvents or exposure to radiations.  

So if we heat a steak of protein, it loses its native structure and said to be denatured. During this it may also undergoes coagulation.