Mass is invariant; only weight changes with gravity

Steps:

• Mass measures matter quantity, unchanged on Moon; weight = mg decreases by factor of ~6 due to lower g.
• Earth-calibrated scales assuming weight equals mass will underread on Moon if they measure force.
• Comparison balances equate torques or forces on both sides, so reduced g affects them equally, preserving mass ratio.
• Y and Z are comparison-type balances (e.g., beam or equal-arm), functioning correctly; W and X are force-based (e.g., spring).

Why D is correct:

• Y and Z compare masses directly; equal masses balance regardless of g, per torque equilibrium (τ = rF sinθ, F ∝ m on both pans).

Why the others are wrong:

• A: W and X both rely on weight force, underreading mass by g ratio.
• B: W relies on weight, incorrectly scaling Z's valid comparison.
• C: X relies on weight, incorrectly scaling Y's valid comparison.

Final answer: D