The lock and key mechanism refers to

The mechanism of enzyme action.

Explanation:

The lock and key mechanism refer to the mechanism of enzyme action. The enzyme is a bio catalyst which enhance the biochemical reactions without changing itself. In the lock and key mechanism, a specific substrate will attach to the active site of the enzyme to form an enzyme-substrate complex. It's like a specific key fits with the lock. The active site is the portion of the enzyme, where it gets attached to the substrate. Then the substrate becomes the product and detaches from the enzyme. Now the enzyme becomes free and another specific substrate will attach to carry on the enzyme action.

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