Final answer:
Trypsin is the correct answer for a serine protease, which plays a crucial role in protein digestion by breaking down proteins into peptides.
Step-by-step explanation:
The question asks which of the listed options is a serine protease. Option (c) trypsin is the correct answer.
Trypsin is an enzyme that catalyzes the hydrolysis of proteins, breaking them down into peptides during the process of digestion. It is produced in the pancreas and is one of the key enzymes involved in the chemical digestion of proteins.
On the other hand, factors XIa and IXa are involved in the blood coagulation cascade, whereas subtilisin is a serine protease produced by bacteria.