To answer this question, let's go through each option to understand why it is correct or incorrect:
Option A) a - This option is incorrect because the verb "regulating" is correctly used in the sentence to describe one of the duties of a police man.
Option B) b - This option is incorrect because the verb "investigating" is correctly used in the sentence to describe one of the duties of a police man.
Option C) c - This option is correct because the phrase "to contain anti-social elements" is grammatically incorrect. The correct phrase should be "to combat anti-social elements" or "to deal with anti-social elements."
Option D) d - This option is incorrect because there is an error in the sentence, as explained in Option C.
The correct answer is C.