The correct answer is: C. Either I or II is implicit.
The statement “I have written several letters to the branch manager regarding my account in the bank but did not receive any reply so far” does not provide enough information to determine whether or not the branch manager is expected to read letters received from the customer or to reply to the letters received from the customers. It is possible that the branch manager is expected to do both, but it is also possible that the branch manager is only expected to do one or the other. Additionally, it is possible that the branch manager is not expected to do either.
Therefore, either assumption I or assumption II could be implicit in the statement.