Each bulb rating: 4V, 6W. Resistance of each bulb = V²/P = 16/6 = 8/3 Ω.
For safe operation, voltage across each bulb must not exceed 4V.
Number of bulbs needed = Total supply voltage ÷ Voltage per bulb = 240/4 = 60.
(c) 60
The key idea: bulbs in series share the supply voltage equally. Each bulb is rated for 4V, so the number of bulbs = 240V ÷ 4V = 60. This ensures each bulb gets exactly its rated voltage (4V) and operates at desired brightness without damage. The power/resistance calculation is not required here — just the voltage division in series.