The correct option is
C Who is responsible for the faults?
Option A - As the given sentence has a question mark, it is an interrogatory sentence, it should begin with the question word 'who'. Auxiliary 'is' should be placed after 'who' to determine the state of being. 'For the faults' is the objective clause, which should be placed at the end of the question. Hence, option is incorrect.
Option B - As the sentence is an interrogatory sentence, we should begin the question with a question word and not the object. Hence, option B is incorrect.
Option C - Though 'is' is used as an auxiliary question word, we must use 'who' as it is the question word in the given sentence and 'is' is just an auxiliary verb. Hence, option C is incorrect.
Option D - The given sentence begins with a question word and is accompanied by the auxiliary 'is' and the verb 'responsible'. The objective clause 'for the faults' is in its place. Hence, option D is the correct answer.
'Who is responsible for the faults?'