Torque balance about the hand's support point Steps:

• The center of mass (CM) is at the geometric center of the uniform rectangular book.
• With longer sides vertical, the horizontal width is b; CM is b/2 from the hand's vertical edge support.
• Weight W acts downward at CM; its line of action is vertical, so lever arm is horizontal distance b/2.
• For equilibrium, hand exerts torque equal in magnitude to W × (b/2), clockwise as per diagram convention.

Why A is correct:

• Torque magnitude is W times lever arm to CM (b/2); clockwise direction matches the balancing torque on the book.

Why the others are wrong:

• B: Correct magnitude but wrong direction (anticlockwise instead of clockwise).
• C: Twice the correct magnitude (uses full b instead of b/2 to CM).
• D: Uses vertical dimension a (irrelevant); torque requires horizontal lever arm b/2.

Final answer: A