Auseful model for enzyme action is the:
a. double helix model.
b. amino acid chain model.
c. ball and socket model.
d. the lock and key model.

A useful model for enzyme action is the  Lock and Key model

 Option D

Explanation:

Enzymes are nothing but chemicals which are used to increase the reaction rate such that the reaction gets completed at faster rate. The working mechanism of enzyme can be best understood by the lock and key model which was put forth by Emil Fischer in 1890.

In this model he explains about the relation between the dynamic site of an enzyme and a substrate particle.  The dynamic site was thought to have a fixed structure (the lock), which precisely coordinated the structure of a particular substrate (the key).

When the correct substrate fits in the enzyme it causes enzyme and substrate to interact with each other. The substrate is changed over to products and is subsequently discharged, freeing the protein.

the answers are a and c