Pepsinogen is converted into stomach in:

Pepsinogen, on the other hand, is the inactive precursor that is secreted by the chief cells of the stomach. When it is exposed to the brutal acidic environment of the stomach, which is brought about by hydrochloric acid (HCl), it is converted to the enzyme pepsin, which is responsible for the breakdown of proteins. The HCl, which has a low pH, brings the pH of the solution to between 1.5 to 3.5, which causes the change of shape of pepsinogen to pepsin. Once there is a small amount of pepsin, it will catalyze the conversion of more pepsinogen to pepsin, which is referred to as autocatalysis.

• Option (d) Pepsin is not correct. This option is not correct because pepsin is an enzyme, not the initial substance that activates pepsinogen to pepsin. Pepsinogen, being the inactive form of pepsin, is activated initially by hydrochloric acid (HCl). When some pepsin is produced, it catalyzes the production of more pepsin from pepsinogen, but the initial activation of pepsinogen to pepsin occurs with HCl, making option (a) the correct option, not option (d).