You would cut the mirepoix ingredients smaller when making a fish fumet than when making a beef stock because the

a. beef stock cooks faster
b. fumet requires less cooking time
c. beef stock is darker
d. fish has a more delicate flavor

b. fumet requires less cooking time.

Explanation:

Fish fumet is concentrated fish stock. The difference between making a fish stock and a beef stock is that the fish fumet requires less cooking time. Because of this, any ingredients that are added to it have to be cut smaller than those that are involved in the beef stock. This ensures that the ingredients are fully cooked without being overcooked.