A50kg box is being pushed along a horizontal surface. the coefficient of kinetic friction between the box and the ground is 0.35.what horizontal force must be exerted on the box for it to accelerate at 1.20m/s^2

For Newton's second law, the resultant of the forces acting on the box is equal to the product between the mass of the box m and its acceleration a:
 
We are interested only in what happens on the x-axis (horizontal direction). Only two forces act on the box in this direction: the force F, pushing the box along the surface, and the frictional force  which has opposite direction of F (because it points against the direction of the motion). Therefore we can rewrite the previous equation as

and solve to find F:

Iwannasay c is the most accurate