Work equals gravitational potential energy increase when stacking blocks

Steps:

• All blocks start with centers at height h/2 above the table.
• In the final stack, centers are at h/2 (bottom), 3h/2 (second), 5h/2 (third), and 7h/2 (top).
• Height changes for centers: 0 (bottom), +h (second), +2h (third), +3h (top).
• Total work = mg × (0 + h + 2h + 3h) = 6mgh.

Why B is correct:

• It equals the total rise in potential energy, per conservation of energy for lifting against gravity.

Why the others are wrong:

• A: Ignores lifts for second and third blocks fully.
• C: Overcounts by adding unnecessary height for bottom block.
• D: Exaggerates total by including extra fictional lifts.

Final answer: B