Useful work equals gravitational potential energy gain

Steps:

• Identify useful work as energy transferred to increase the stone's potential energy.
• Recall potential energy change = mass × gravity × height = m g h.
• Note weight W = m g, so m g h = W × h.
• Ramp length L is irrelevant for useful work, which depends only on vertical height h.

Why none of the options are correct:

• The expression for useful work is W × h, not listed.

Why A is wrong:

• m × h omits g, so it's W h / g, not work (units incorrect).

Why B is wrong:

• W / h is weight divided by height, yielding force/length, not work energy.

Why C is wrong:

• m × l uses mass and ramp length, ignoring g and vertical displacement.

Why D is wrong:

• W × l is total work along ramp (including inefficiency), not useful vertical work.

Not enough information to match options to W × h.

Final answer: None (useful work = W × h)