Archimedes' Principle Determines Upthrust

Steps:

• All cuboids have identical length, breadth, and height, so same volume $V$.
• Held fully immersed at same depth in identical orientations, each displaces volume $V$ of water.
• Upthrust $F_b = \rho V g$, independent of cuboid density.
• Thus, $F_b$ is identical for W, X, Y, and Z.

Why A is correct:

• Archimedes' principle states buoyant force equals weight of displaced fluid; identical displaced volume yields same upthrust.

Why the others are wrong:

• B: Upthrust on W equals that on X, not twice.
• C: Upthrust on Y equals that on W, not twice.
• D: Upthrust on Z equals that on X, not twice.

Final answer: A