Answer:
d. f
Step-by-step explanation:
Based on the principle of operation of transformer, when a.c voltage is applied at the terminal of the primary coil (input coil), an alternating magnetic flux is produced in the iron core which links the secondary coil (output coil).
Also, an alternating e.m.f of the same frequency as that of primary coil (input coil) will be induced in the secondary coil (output coil) by mutual inductance.
Thus, If the input frequency is "f" then the output frequency will be "f" (since the frequency will be the same).
The correct option is "D"