Manuel is holding a 5kg box. how much force does each of his legs exert

3 × 10 N

Explanation:

The weight of the box can be calculated using Newton's second law of motion.

F = m . a

where,

F is the force (in this case, the weight of the mass)

m is the mass of the box

a is the acceleration due to the gravity

W = 5 kg × (-9.8 m/s²) = -5 × 10 N

According to Newton's third law of motion, the force that the box exerts on Manuel is counteracted by the force Manuel exerts on the box, that is, they have the same magnitude and orientation but opposite sense. The force that Manuel exerts is 5 × 10 N. If each leg exerts half of this force, the force exerted by each leg is 1/2 × 5 × 10 N = 3 × 10 N (we round off to 1 significant figure).

think its the third option