The correct option is A 1000 km/hr
Speed of the jet airplane with respect to ground is, Vjet,g=500 km/h .
Relative speed of its products of combustion with respect to the plane.
Vsmoke,jet=−1500 km/h
Speed of its products of combustion with respect to the ground =Vsmoke, grd
Relative speed of its products of combustion with respect to the airplane,
Vsmoke, grd=Vsmoke,jet+Vjet, grd
Vsmoke, grd=−1500+500
Vsmoke=−1000 km/h