The surest way of stopping

[komati_anusha]

The surest way of stopping the manufacture of methamphetamine is the requirement
for a prescription for its essential ingredient, pseudoephedrine.

A. The surest way of stopping the manufacture of methamphetamine is the
requirement for a prescription for its essential ingredient, pseudoephedrine.
B. The surest way to stop the manufacturing of methamphetamine is requiring a
prescription for its essential ingredient, it being pseudoephedrine.
C. The surest way to stop the manufacture of methamphetamine is to require a
prescription for its essential ingredient, pseudoephedrine.
D. The surest way of stopping the manufacture of methamphetamine is requiring
of a prescription for their essential ingredient, pseudoephedrine.
E. The surest way to stop the manufacture of methamphetamine has been to
require a prescription regarding its essential ingredient, which is
pseudoephedrine.

[komati_anusha]

5sec Scan - Couldn't identify the Error Type
Eliminate
A - Too wordy ( Of, for )
B - it being ( Avoid )
D - their ( only for people )
C -?
E - ,which ( referring to ingredient )
So chosen E.
Why OA is C?

Rich, Can you please explain