Right option is (b) When flow is sonic and mass flow remains constant at the throat despite reducing exit pressure
The best I can explain: When we reduce the exit pressure, there comes a point when the Mach number becomes 1 at the throat i.e. the flow is sonic. The mass flow remains constant and the flow becomes frozen upstream of the throat. This condition where after reaching sonic flow at the throat, and despite reducing the exit pressure, the mass flow remains constant is called choked flow.