Your bacterium is growing on a type of medium called casein agar, which contains milk protein (casein). There is a clear zone around the growth area of the bacterium, showing that it is synthesizing the enzymes needed to catalyze the breakdown of casein. These enzymes are considered:
a. apoenzymes.
b. exoenzymes.
c. ribozymes.
d. regulated enzymes.
e. endoenzymes.

b. exoenzymes.

Explanation:

Casein (Phosphoprotein) is found in milk of mammals like comprises 80% of cow milk. Some bacteria possess the ability to breakdown the casein they produce the clear zone around the growth area of the bacteria in the medium while other bacteria produce the opaque and white.

Casease is the enzyme that breakdown the casein by its proteolytic activity into some bacteria. It convert the casein protein into smaller polypeptides and amino acids.

Casease is an exoenzyme because it is secreted by the bacterial cell into environment and works outside the bacterial cell. While endoenzymes are those that works inside the cell.