Using the following:
(a) constant acceleration
(b) variable acceleration
(c) variable speed and constant velocity
(d) constant retardation
The acceleration vector lays always in the line that connects the centre of the circular path and the point where the body is. So for each point of the circumference, the vector will be different. This means (a) is false, and (b) is true.
Speed is the scalar magnitude, the modulus of the velocity vector. Speed, by the question itself, is constant, while the velocity vector changes: it always lays on a tangent line to the circular path. This means (c) is false.
Also, since speed is constant, it means the acceleration doesn't change the speed. From this we deduce that the acceleration doesn't decrease the speed, so there's no retardation, which means (d) is false.