The correct answer is 2) or 3)

A nozzle designed for low altitude (high back pressure) will not produce as much thrust at high altitude (low back pressure) as a nozzle specifically designed for low back pressure. As a result, rocket nozzles for launch vehicles are typically designed to operate best at an intermediate pressure. Thus at launch, they see a higher backpressure than ideal (resulting in oblique shocks in the exit plane). At high altitudes (or in space) they typicallly see a lower back pressure than ideal. For these conditions, the flow continues to expand upon leaving the nozzle resulting in an expansion fan.

Class performance (2003):

Class performance (2001):