Starting from rest, a 4.0-kg body reaches a speed of 8.0 m/a in 20 s. what is te net force acting on the body?

The guy below is wrong!

F=ma
Where force = mass x acceleration

We dont have acceleration, a= change in velocity divided by the time taken.
a = v (final velocity) - u (initial) / t
a us 8-0 (at rest means u was 0) / 20 = 0.4

Using F=ma

F= mass x acceleration 
F= 4 x 0.4
F=1.6 N

B. convex since the words are bigger