The primary winding of a power transformer should always be

ANS:D - fused

The Correct option is D, fused.

When the secondary load is overloaded, it draws maximum current in the primary side due to which damaging the primary side as well as appliances. Therefore primary winding of power transformer is fused. Suppose peak voltage of power source is varying then current will also fluctuate so also to limit the current it is fused.