Short answer
My best bet is D)
Background
A) seems incorrect as lipases break down fats. However, one has to realize there quite some fat in milk (up to 3.5% in untreated milk). Digestive lipases in man are secreted by the pancreas into the duodenum.
B) Might be a correct answer, as trypsin is released in the small intestine and...
[Trypsinized] peptide products are further hydrolyzed into
amino acids via other proteases, rendering them available for
absorption into the blood stream. Tryptic digestion is a necessary
[first] step in protein absorption as proteins are generally too large
to be absorbed through the lining of the small intestine.
C) Chymosin, known also as rennin, is a proteolytic enzyme synthesized by chief cells in the stomach. Its role is thought to be in digestion is to curdle or coagulate milk in the stomach, a process important in young animals. If milk were not coagulated, it would rapidly flow through the stomach, impairing protein digestion. So this is a enzyme in the stomach and active earliest in the digestive system (answers A and B concern enzymes in the duodenum, which is later down in the digestive tract). However, in humans there is only a chymosin pseudogene present, and since the answer specifically deals with humans, answer C) is incorrect.
D) Pepsin is
...[] one of three principal proteases in the human digestive system, the other two being chymotrypsin and trypsin. [These] enzymes [...] break down dietary proteins into [...] amino acids, which can be readily absorbed by the small intestine. Pepsin [mainly] cleav[es] peptide bonds between hydrophobic and [..] aromatic amino acids [...The other two proteases have different targets].
Pepsin's proenzyme, pepsinogen, is released [...] in the
stomach [...], and upon mixing with the [...]
gastric juice, [it is] activate[d] to [...] pepsin.
Here we have the adult equivalent of rennin, also active early in the digestive tract, namely the stomach. So D) is the enzyme present in humans (as opposed to C) and active already in the stomach (as opposed to later in the duodenum in answers A and B). So D is the answer.
