A 200 V/100 V, 50 Hz transformer is to be excited at 40 Hz from 100 V side. For the excitation current to be same, the applied voltage should be

The excitation current in a transformer is primarily determined by the magnetizing reactance, which is inversely proportional to frequency and directly proportional to the core properties and the number of turns. For a constant excitation current, the magnetic flux density in the core must remain roughly constant. This means the $V/f$ ratio must be kept constant.