A ball hits a vertical wall horizontally at 10 m/s bounces back at 10 m/s

• A

  There is no acceleration because $10\frac{\mathrm{m}}{\mathrm{s}}-10\frac{\mathrm{m}}{\mathrm{s}}=0$

• B

  There may be an acceleration because its initial direction is horizontal 

• C

  There is an acceleration because there is a momentum change

• D

  Even though there is no change in momentum there is a change in direction. Hence it has an acceleration