Answer: C. Translational equilibrium of the box gives $N - mg - 10 = 0$ so the platform’s force on the box is $N = (3.0)(10)+10 = 40\ \text{N}$ By Newton’s third law, the box exerts an equal-magnitude force on the platform.

A: Treats the third-law forces as cancelling, although they act on different objects, giving $0\ \text{N}$
B: Balances only the box’s weight, omitting the person’s downward force, giving $30\ \text{N}$
D: Counts the person’s downward force twice, giving $30+2(10)=50\ \text{N}$
Remember: sum forces on one object for equilibrium; third-law pairs act on different objects.