Work to stack blocks equals gravitational potential energy gained by their centers of mass

Steps:

• The bottom block requires no work as it stays on the table.
• The second block is lifted by h (its center of mass rises h), so work = mgh.
• The third block is lifted by 2h (center of mass rises 2h above table), so work = 2mgh.
• The fourth block is lifted by 3h (center of mass rises 3h above table), so work = 3mgh.
• Total work = mgh + 2mgh + 3mgh = 6mgh.

Why B is correct:

• B matches the total gravitational potential energy increase, where each block's work is mgh times the height its center of mass is raised.

Why the others are wrong:

• A: Underestimates by omitting full stacking heights for upper blocks.
• C: Overestimates, possibly by adding unnecessary extra heights.
• D: Overestimates further, likely from incorrect summation like 1+2+3+4.

Final answer: B