Answer: C. 3.75 seconds
In this case, the initial speed/velocity is 20m/s and the final velocity is 35m/s with 4m/s2 acceleration. To determine how long it takes the bike to the final velocity you need to divide the difference in velocity(final velocity-initial velocity) with acceleration. The equation would be
time= (final velocity - initial velocity) / acceleration
time= (35m/s-20m/s) /4m/s2 = 15/4 s= 3.75s