The answer is (c): the same as the voltage induced in the square coil, as seen in the photographs below. In the photograph at the left, current in the round coil produces a changing magnetic field, inducing a voltage in the square coil. In the photograph at the right, current in the square coil produces a changing magnetic field, inducing a voltage in the round coil.
Can you see the difference? Neither can I.
According to the idea of mutual induction, either of the two coils can be used as the primary, in which the current flows, or the secondary, in which the voltage is induced. The situation is entirely symmetric.
