Step 1: Picture proteins as chains of beads.
A protein is like a long chain, and each bead on that chain is a single amino acid. Different chains use different amino acids in different orders, and that order decides what the protein does.
Step 2: Count the types of beads available.
Out of the many amino acids found in nature, only a set of 20 are called standard amino acids because they are the ones cells actually use to build proteins. Some of these the body makes itself, and some, called essential amino acids, must come from food.
Step 3: Rule out the other numbers.
Ten and fifteen fall short of the real count biologists have identified. Twenty five goes past it. None of these numbers match the standard set recognized in cell biology.
Step 4: Settle on the right number.
Since science recognizes 20 standard amino acids as the building blocks of every protein, that is the correct count.
The correct answer is 20.