Ans: The correct answer is: option (d)
One litre of water at 300C is mixed with one litre of water at 500C. The temperature of the mixture will be between 300C and 500C. This is because there is transfer of heat from an object with high temperature to an object having low temperature. So, heat will flow from one litre of water at 50◦C to one litre of water at 300C. So, the temperature of the mixture will be between 300C and 500C.