Velocity is measured by (final velocity - initial velocity)/time taken, or simply put, change in velocity/time taken. Our original velocity was 8, and our final was 22.
With our information, subtract 22 by 8 and then divide by 7 (final - inital)/(time taken).
22-8 = 14
14/7 = 2.
Your average acceleration was 2.0 m/s^2.
The answer is D.)
I hope this helps!