The induced fit model of enzyme action fine tunes the original concept of the lock and key model and modifies it. this induced fit model changes the original explanation by hypothesizing that

The correct answer is - the active site of the enzyme-modified so it can fit the substrate and begin the chemical reaction.

Explanation:

The induced-fit model of the enzyme is a hypothesis that tries to explain the interaction between enzyme and substrate.

It is a theory that refines lock and key theory by suggesting that if an enzyme comes in touch with a particular substrate, the active site of enzyme molds or changes its shape to fit the substrate. The properly fit enzyme-substrate complex initiates the chemical reaction.

Thus, the correct answer is - the active site of the enzyme modified so it can fit the substrate and begin the chemical reaction.