The enzyme that breaks down starch is:

Amylase is the enzyme specifically responsible for breaking down starch into simpler sugars, such as maltose and glucose. It accomplishes this by hydrolyzing the glycosidic bonds between the glucose units in starch. This process primarily begins in the mouth, where salivary amylase is secreted, and continues in the small intestine, where pancreatic amylase further breaks down starch.

The other enzymes listed do not perform the same function. Protease breaks down proteins into peptides and amino acids, lactase is involved in the digestion of lactose (the sugar found in milk), and cellulase breaks down cellulose, a component of plant cell walls. Thus, amylase is the key enzyme for starch digestion, making it the correct answer.