The correct option is C (c)
Let’s examine the sentence: “(a) The thief (b) hit me (c) suddenly and hardly.” The sentence wants to convey that the thief hit the speaker in a very hard manner. However, ‘hardly’ does not convey this meaning as it means ‘barely’ or ‘not at all.’ Hence, ‘hardly’ should be replaced with ‘hard.’ Hence, the error is in Part C of the sentence. So, Option C is the correct answer.