Explanation:
A Carnot engine is a theoretical engine that operates on the Carnot cycle, which is a reversible cycle made up of four processes: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression. The efficiency of a Carnot engine is given by the formula:

Efficiency = 1 - (Tc/Th)

Where Tc is the temperature of the cold reservoir (sink) and Th is the temperature of the hot reservoir (source).

Effect of increasing source temperature:
When the temperature of the source is increased while keeping the sink temperature fixed, the difference between the source temperature and the sink temperature (Th - Tc) increases. As a result, the efficiency of the Carnot engine also increases.

Reason behind the increase in efficiency:
The increase in the temperature difference (Th - Tc) leads to an increase in the heat input to the engine. This is because the heat transfer is directly proportional to the temperature difference. As a result, more heat energy is available to do work, resulting in an increase in efficiency.

Significance of fixed sink temperature:
The efficiency of a Carnot engine depends on the temperature difference between the source and the sink. If the sink temperature is also increased along with the source temperature, the temperature difference remains the same, resulting in no change in efficiency.

Dependence on working fluid:
The efficiency of a Carnot engine is independent of the working fluid used. It solely depends on the temperatures of the source and the sink.

Conclusion:
In conclusion, if the temperature of the source is increased while keeping the sink temperature fixed, the efficiency of the Carnot engine increases. This is due to the increased temperature difference, which allows for more heat energy to be converted into work. The efficiency of the Carnot engine is independent of the working fluid used but depends solely on the temperatures of the source and the sink.