Here, Mr. A caused apprehension of death to Mr. B by pointing gun to Mr. B.
So, here C (who was standing behind B) has the right to defend B. According to Sec. 96 of Indian Penal Code.
But, when there is apprehension of death of a person. A person himself or any other person has the right of private defence which can extend to death, if the death is otherwise be the consequence of the apprehension.
Hence, Mr. C has the right of private defence which can extend to death of Mr. A to defend Mr. B from the assault of Mr. A.
-. Accordingly, Mr. C can take the advantage of private defence.