Calculate voltages across series resistors P and Q to find total emf

Steps:

• Voltage across P: V_P = power / current = 18 W / 2 A = 9 V.
• Voltage across Q: V_Q = energy dissipated / charge = 24 J / 4 C = 6 V.
• Circuit is series (implied by single current through P and charge through Q), so total emf = V_P + V_Q.
• Emf = 9 V + 6 V = 15 V.

Why D is correct:

• Emf equals sum of potential differences across resistors in series, per Kirchhoff's voltage law.

Why the others are wrong:

• A: 3 V ignores total circuit voltage.
• B: 6 V matches only V_Q.
• C: 9 V matches only V_P.

Final answer: D