Rate of change of momentum Steps:

• Mass flow rate of water: ρ A v
• Horizontal momentum inflow per unit time: (ρ A v) v = ρ A v²
• Water stops horizontally upon hitting wall (velocity change = v)
• Force on wall equals momentum change rate: ρ A v²

Why D is correct:

• Newton's second law for fluids gives force as mass flow rate times velocity change, yielding ρ A v²; D matches the ρ v² dependence (multiplied by area A).

Why the others are wrong:

• A: Uses v instead of v², ignoring squared velocity in momentum.
• B: Divides by A, yielding pressure rather than total force.
• C: Lacks v² term, underestimating momentum contribution.

Final answer: D