Four different candles, which can last for 5 hours, 4 hours. 3 hours and 2 hours of burning, respectively, are lit in a room at the same instant and allowed to burn till such time that the three candles in the room get extinguished. The cost of burning each one of these candles is 75 paise per hour. The cost involved is:

To solve this problem, we need to determine the total burning time for all four candles until three of them are extinguished.

The four candles have lifespans of 5, 4, 3, and 2 hours. Since they are all lit at the same instant, they burn simultaneously.

The condition states we must stop the process when three candles get extinguished. This happens at the end of the 4th hour, as by then, the candles that last 2, 3, and 4 hours will have finished burning.

During these 4 hours: The 2 hour candle burns for its full 2 hours. The 3 hour candle burns for its full 3 hours. The 4 hour candle burns for its full 4 hours. The 5 hour candle burns for 4 hours before the process stops.

Total burning time = 2 plus 3 plus 4 plus 4 = 13 hours.

The cost is 75 paise per hour. Total cost = 13 hours multiplied by 0.75 Rupees = 9.75 Rupees.

Therefore, the correct option is C.