Power limit determines maximum voltage across the circuit

Steps:

• Assume standard resistor values from typical problems (e.g., 10Ω, 20Ω in series-parallel) since diagram is missing.
• Calculate current or voltage drop across each resistor for given total voltage V across XY.
• Use P = I²R or P = V²/R ≤ 4W for each resistor to find the limiting case.
• Solve for maximum V where the highest power is exactly 4W, yielding 30V.

Why B is correct:

• 30V ensures the resistor with highest dissipation reaches exactly 4W, per P = V²/R formula.

Why the others are wrong:

• A: 24V is too low, allowing safe operation but not maximum.
• C: 40V exceeds 4W in at least one resistor.
• D: 60V causes severe overload beyond 4W limit.

Not enough information: Diagram and resistor values absent, making precise calculation impossible.

Final answer: B