In head on elastic collision of two bodies of equal masses (a) the velocities are interchanged

The correct answer is all

Explanation: 

If u and v are the velocities before collision and u' and v' are the velocities after collision, then we have 

u' = (m-m)u/(m+m) +2mv/(m+m) = 0 + v = v and v' = 2mu/(m+m) + (m-m)v/(m+m) = u + 0 = u 

So the velocities and speeds are interchanged. Hence (a) and (b) are true. 

Since the velocities are interchanged and masses are equal hence the momenta are also interchanged. Hence (c) is true. 

If u > v then after the collision the speeds of bodies are interchanged. Now the faster body slows down and the slower body speeds up. Hence (d) is true.