(a) F = 250 kg × g = 250 × 10 = 2500 N s = 1 m
W = F. s = 2500 N m = 2500 J
(b) Zero, as the box does not move at all while holding it.
(c) In order to hold the box men are applying a force which is opposite and equal to the gravitational force acting on the box. While applying the force men make muscular effort. So they get tired.