The correct answer is C) substrate; active site.
The other options of the question were A) product; substrate. B) active site: product. D) active site; substrate.
In the Lock and Key model of enzyme action, the substrate fits into the active site of the enzyme.
It was scientist Emil Fisher in 1874 who proposed this comparison in which an enzyme is the lock enters the substrate is the key. Fisher established that the appropriate key or substrate was going to fit in the correct hole or active site. This active site has to have a specific geometric shape to fit.
The substrate fits into the action site of the enzyme.