Examine the graph

First we need to understand that what is Uniform and Non-uniform motion , that is ,

Uniform motion – When a body travels equal distance in equal intervals of time then it is called uniform motion which simply means that the velocity of the object must remain constant in this type of motion.

Non Uniform motion – When a body travels unequal distance in equal intervals of time then the motion traced by the body is called non-uniform motion which simply means that the body possesses some constant acceleration or at extreme cases it changes it’s acceleration with time.

a). If we consider the full motion of the object that is from t=0s to t=6s , then we can say that the motion of the object is non uniform motion , even if we choose some portions of the graph then also it is non-uniform motion because there is no point where the velocity of the object is constant. Hence full motion is non-uniform.

b). UNIFORMLY ACCELERATED MOTION – It means that when body changes equal amounts of velocity in equal intervals of time then it is called uniformly accelerated motion which simply means that the acceleration of the body should remain constant.

If we divide the full motion into two parts and then look at them one at a time , the parts should be first from t=0s to t=4s(point A) and second the rest from point A to point B , Then the two motions are called uniformly accelerated motions. So , yes from t=0s till point A the object is in uniformly accelerated motion.

Hence from point A to point B the object now also follows uniformly accelerated motion but with different magnitude of acceleration which was in t=0s till point A. Hence this time it has deceleration or retardation which means negative of acceleration simply which reduces velocity and at the end Point B the object is at rest.