Buoyant force depends on displaced volume, constant when fully submerged

Steps:

• Upthrust equals weight of displaced seawater, proportional to submerged volume.
• For lower face depth $d < a$ (cube side $a$), partially submerged; volume = base area $\times d$, upthrust increases linearly with $d$.
• At $d = a$, fully submerged; volume = cube volume, upthrust reaches maximum.
• For $d > a$, fully submerged with constant volume displaced, upthrust constant.

Why C is correct:

• Graph C shows linear increase to $d = a$ then constant, matching Archimedes' principle for submersion phases.

Why the others are wrong:

• A: Constant throughout, ignores partial submersion increase.
• B: Decreases with depth, contradicts constant full submersion.
• D: Continues increasing beyond $d = a$, violates fixed displaced volume.

Final answer: C