Torque from a couple of parallel forces separated by width b

Steps:

• Identify the two forces of magnitude F acting along parallel edges of the rectangle, separated by distance b.
• Confirm the forces are equal, opposite, and parallel, forming a couple.
• The torque of a couple equals force magnitude times perpendicular distance between lines of action.
• Thus, net torque about P is F × b.

Why B is correct:

• For a couple, torque = F × d where d = b, the separation between parallel edges, per rigid body torque formula.

Why the others are wrong:

• A: Assumes separation a, but diagram shows b as the relevant distance.
• C: Incorrectly doubles Fa, ignoring couple reduction to single F times separation.
• D: Overestimates by factor of 2, as couple torque is F b, not 2 F b.

Final answer: B