The correct answer is A. core built-up of laminations of cold rolled grain oriented steel.
Laminations are thin sheets of steel that are stacked together to form the core of a transformer. The laminations are insulated from each other by a thin layer of varnish or lacquer. This insulation prevents eddy currents from flowing in the core, which would cause losses.
Cold rolled grain oriented steel is a type of steel that has been specially treated to align the grains of the metal in a specific direction. This alignment makes the steel more magnetic, which increases the permeability of the core.
Hot rolled sheet steel is a type of steel that has been rolled at high temperatures. This process causes the grains of the metal to be randomly oriented. This random orientation makes the steel less magnetic, which decreases the permeability of the core.
Therefore, the core of a transformer will have lower core losses and higher permeability if it is made from cold rolled grain oriented steel.