Which enzyme is likely to be the most active in the acidic environment of the stomach?

Stomach acid  activates an enzyme called pepsinogen which then becomes pepsin that is secreted by the stomach wall. This enzyme is only active within the pH range of 3.0 to 5.0 and requires the acid to maintain that pH.

the cell theory states that all living things including animals, as well as plants, are made of cells.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.