What do A, B, C and D represent in the following figure?

Carrier proteins are integral membrane proteins that exist within and span the membrane across which they transport substances. Antiport transports two molecules/anions/cations at once through the membrane in opposite directions. Symport transport moves two molecules/anions/cations simultaneously across a cell membrane in the same direction. Uniport transport mechanism drives a single ion across a membrane and not coupled with transport of any other compound or ion. So, the correct answer is 'A : Carrier protein, B : uniport C : antiport, D : symport'