. . . . . . . . you read, you cannot pass. A. Till B. Unless C. When D. If

The correct answer is if.

The word “if” is used to introduce a condition or supposition. In this case, the condition is that the student must read in order to pass.

The word “till” is used to indicate a point in time up to which something happens. In this case, it would not make sense to say that the student must read “till” they pass, because they could read for a long time and still not pass.

The word “unless” is used to introduce a condition that must be met in order for something to happen. In this case, it would not make sense to say that the student must read “unless” they pass, because they cannot pass unless they read.

The word “when” is used to indicate a point in time at which something happens. In this case, it would not make sense to say that the student must read “when” they pass, because they cannot pass until they read.