Force balance at terminal velocity

Steps:

• At constant speed, net force on object is zero.
• Downward force is object's weight $W_o$.
• Upward forces are buoyancy (weight of displaced liquid $W$) and viscous upthrust $F_u$.
• Balance gives $W_o = W + F_u$, so $F_u = W_o - W$.

Why A is correct:

• It follows Newton's first law and force equilibrium at constant velocity, where viscous upthrust equals the net downward force after buoyancy.

Why the others are wrong:

• B: Implies upthrust opposes buoyancy, violating force directions.
• C: Adds weights, exceeding total downward force.
• D: Sums unrelated upward contributions, ignoring balance.

Final answer: A