When oxygen is present

a. most cells utilize aerobic cellular respiration.
b. most animal cells will carry on fermentation and produce lactic acid.
c. most bacteria and yeasts carry on fermentation.
d. two ATP molecules are produced for each glucose molecule.
e. most animals will convert CO2 to glucose.

To answer this question, we can eliminate option e. right away, as animals do not convert CO2 to glucose.

Next, let's consider option b. The statement that most animal cells will carry on fermentation and produce lactic acid is incorrect. In the presence of oxygen, animal cells typically undergo aerobic cellular respiration rather than fermentation.

Option c. states that most bacteria and yeasts carry on fermentation. While it is true that fermentation is a common metabolic pathway in these organisms, it does not necessarily mean that it occurs in the presence of oxygen. Some bacteria and yeasts can switch between different metabolic pathways depending on the availability of oxygen.

Option d. mentions that two ATP molecules are produced for each glucose molecule. This statement is not accurate. In aerobic cellular respiration, which occurs in the presence of oxygen, a total of 36 to 38 ATP molecules are produced per molecule of glucose.

This leaves us with option a. It is the correct answer. When oxygen is present, most cells, including animal cells, utilize aerobic cellular respiration. This process involves the breaking down of glucose to produce energy in the form of ATP, and it requires the presence of oxygen.

So, the correct answer is a. most cells utilize aerobic cellular respiration when oxygen is present.