Steady current conservation

Steps:

• Current I is constant throughout the wire in steady state, so charge flow rate is uniform.
• Charge Q passes through R in time t, so I = Q/t.
• In time t/4, charge through any point, including S, is I × (t/4).
• Substitute I: charge = (Q/t) × (t/4) = Q/4.

Why B is correct:

• By conservation of charge in steady current, the same current I flows through all cross-sections, so charge in time t/4 is Q/4 regardless of area change.

Why the others are wrong:

• A: Ignores constant current; area ratio doesn't halve the charge flow rate.
• C: Would apply to full time t, not t/4.
• D: Applies to full time t, not t/4.

Final answer: B