The box is accelerated from rest to 4 m/s in a matter of 2.5 s, so its acceleration a is such that
4 m/s = a (2.5 s)  →  a = (4 m/s) / (2.5 s) = 1.6 m/s²
Then the force applied to the box has a magnitude F such that
F = (10 kg) (1.6 m/s²) = 16 N