To solve this problem, we need to understand the role of the enzyme enterokinase in the digestive process.$\newline \bullet$Enterokinase is an enzyme secreted by the cells of the duodenum.$\newline \bullet$Its primary function is to convert trypsinogen, an inactive enzyme precursor, into trypsin, an active enzyme.$\newline \bullet$Trypsin is a proteolytic enzyme that plays a crucial role in the digestion of proteins by breaking them down into smaller peptides.$\newline$Let's analyze the options:$\newline \bullet$Option 1: Trypsinogen into trypsin - This is correct as enterokinase specifically catalyzes this conversion.$\newline \bullet$Option 2: Caseinogen into casein - This is incorrect as enterokinase does not act on caseinogen.$\newline \bullet$Option 3: Pepsinogen into pepsin - This is incorrect as pepsinogen is activated to pepsin by the acidic environment in the stomach, not by enterokinase.$\newline \bullet$Option 4: Protein into polypeptides - This is incorrect as enterokinase does not directly convert proteins into polypeptides.$\newline$Therefore, the correct answer is Option 1: Trypsinogen into trypsin.