Question

what is the angle between instantaneous velocity and acceleration during the retarded motion

Answer 1

180 deg for 1D motion
during retardation the acceleration will be in opposite direction to motion.
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Answer 2

The angle between velocity and acceleration during the fall of projectile is 0°< θ < 90°. In uniformly retarded motion , acceleration always act in a direction opposite to the velocity.

Hence angle between between instantaneous velocity and acceleration is 180 degree.

Hope it helps