The proper way to revise the sentence would be:
C. After Percy became a professional cook
Step-by-step explanation:
The way to revise the sentence is to set up the framework for a complex statement in the use of this fragment only which is to form the first of the two or many parts of the complex sentence that is to come after it.
The complex sentence thus made would need to have interdependent parts that do not have complete meanings if they are not in direct relations with each other.
In this context one can see how the statement given here draws out another line into the sentence to make it complex.