Question

what is the maximum efficiency that can be obtained by carnot cycle

Answer 1

Hope it will help u
Since Carnot cycles efficiency totally depends on the source and the sink temperature. 
So, for the maximum efficiency we have to define these two temperatures.
The efficiency of Carnot cycle does not depends on the working fluid.

The efficiency of reversible Carnot cycle is maximum and is the function of temperature only.

Efficiency=(Tsource-Tsink)/Tsource