Correct option is D
The critical pressure ratio of a convergent nozzle is defined as the ratio of the outlet pressure to the inlet pressure of the nozzle, specifically when the mass flow rate per unit area through the nozzle reaches its maximum value. This condition corresponds to the flow becoming sonic at the nozzle exit, leading to a choked flow state.
At this critical condition, the flow is choked (Mach number = 1 at the throat), and any further decrease in the downstream pressure will not increase the mass flow rate.
