Answer:
A. increase
Explanation:
Stirling cycle having four processes
1.Two processes are constant temperature processes
2.Two processes are constant volume processes
The efficiency of Stirling cycle is same as the efficiency of Carnot cycle.
The efficiency of Stirling cycle given as
[tex]\eta=1-\dfrac{T_L}{T_H}[/tex]
[tex]T_L[/tex]=Lower temperature
[tex]T_H[/tex]=Upper temperature
If we increase then upper temperature while the lower temperature is constant then the efficiency of Stirling cycle will increase because the ratio of lower and upper temperature will decreases.
Therefore answer is
A. increase