Which of the following loss in a transformer is zero even at full load ?

Answer: Option B

Transformers are static electrical devices, meaning they have no moving parts.
Because of this, there are no mechanical losses such as friction or windage losses, which occur only in rotating machines like motors or generators.

Core losses (which include hysteresis and eddy current losses) occur whenever the transformer core is energized — that is, even under no-load conditions. These losses depend on the supply voltage and frequency, not on the load.

Copper losses occur only when the transformer carries load current and vary with the square of the load current.

Since a transformer has no moving parts, the friction loss is zero at all times — even at full load.

Hence, the correct answer is Friction loss.