ANS:A - center-tapped secondary

Transformer having two winding. At the secondary winding, one common terminal is divided the winding into two equal half winding. The 1st half for 0-90 degree (+ve half cycle) and another half for 90-180 degree (-ve half cycle). And that common terminal is called as center tapped terminal. Like that centered tapped transformer is required for 180 degree input to rectify.