Correct choice is (d) Deacylation
To elaborate: Chymotrypsin reaction with its substrates takes place in 2 stages, an initial burst phase at the beginning of the reaction and steady state phase following MM kinetics. The mode of action is explained in 2 steps:
* Acylation of substrate to form an acyl-enzyme intermediate.
* Deacylation in order to return the enzyme to its original state.
Hence the first step is acylation.