Question

the lock - and - key mechanism refers to
the complementary shapes of an enzyme and a substrate.
the attractive forces between an enzyme and a substrate.
the ability of an enzyme to lower the activation energy of a reaction.
the ability of an enzyme to \unlock\ the products of a reaction.

Explanation:

The lock - and - key mechanism is a model in biochemistry. In this model, the enzyme (lock) has a specific shape that is complementary to the shape of its substrate (key). Just as a key fits into a specific lock, the substrate fits into the active site of the enzyme based on their complementary shapes. The attractive forces (e.g., hydrogen bonds, van der Waals forces) are part of what holds the substrate in the active site once the complementary shapes have allowed binding, but the core idea of the lock - and - key mechanism is about the complementary shapes. Lowering activation energy is a general property of enzymes (catalysis) and not specific to the lock - and - key mechanism. The idea of "unlocking" products is not the essence of the lock - and - key model.

Answer:

the complementary shapes of an enzyme and a substrate.