The correct option is
C tell
The sentence indicates that the subjects cannot differentiate between the twins. In other words, they cannot distinguish between them. Thus, the answer must indicate the meaning 'distinguish'.
Option A is incorrect. 'say' indicates the meaning 'to state or speak' and not 'distinguish'. Thus, it does not have the correct meaning.
Option B is correct. 'tell', in this case, indicates the meaning 'to distinguish'. For example, tell them apart from each other. Thus, it has the required meaning and is the correct answer.
Option C is incorrect. 'make' means 'to bring into existence' and not 'distinguish'. Thus, it does not have the required meaning.
Option D is incorrect. 'prove' means 'to establish the truth' and not 'distinguish'. Thus, it does not have the required meaning.