Drift speed depends on current and wire cross-sectional area Steps:

• Cross-sectional area A ∝ d², so new area A₂ = (0.5d)² / d² × A₁ = 0.25 A₁.
• Current I = n e A v, with n (charge carrier density) and e (charge) constant for same material.
• Rearrange to v ∝ I / A.
• New drift speed v₂ = v₁ × (I₂ / I₁) × (A₁ / A₂) = v × 3 × (1 / 0.25) = 12v. Why D is correct:
• Follows directly from the formula I = n e A v, showing v scales with I/A. Why the others are wrong:
• A: Ignores area reduction requiring higher speed for increased current.
• B: Might assume linear diameter scaling instead of quadratic for area.
• C: Fails to account for both current increase and area decrease. Final answer: D