Answer:(B) the load current.
Whenever the transformer is loaded, load current starts to flow in secondary winding which produces secondary flux in the core of transformer. For neutralizing this secondary flux primary winding draws current from source to produce counter flux in the core. But in this phenomenon some of the flux produced in the secondary will not linked with primary winding and similarly some of the flux produced in primary will not link with secondary winding, as all the flux will not pass through the core. Those fluxes pass outside the core and do not link with other winding, are called leakage flux. As this phenomenon of leakage flux comes into picture only when load is connected to the transformer and obviously varies with load.