When cut-off ratio is __________ the efficiency of Diesel cycle approaches to Otto cycle efficiency.

Correct answer is D
cut-off and cut-off ratio are two different terms, there is no cut-off in Otto cycle (i.e. cut-off is zero but cut-off ratio is one because heat is being added at constant volume so the ration of volumes is one). Now in case of diesel cycle, normally there is cut-off i.e. cut-off ratio is greater than one. As this ratio decreases and becomes equal to one (just like Otto cycle), it behaves like Otto cycle and its efficiency approaches to Otto cycle.
Shortly speaking when cut-off is ZERO or cut-off ration is ONE, efficiency of diesel cycle becomes equal to Otto cycle

in open cycle gas turbine power plant, there is no condenser used. the expanded gases are directly sent to atmosphere and not routed back to the same cycle (this is why it is called open cycle turbine power plant), therefore Option D can never be correct answer

Answer is one not zero.