The correct option is B Buoyant force decreases
Buoyant force acting on an airship decreases as it rises in the air as air at higher altitude becomes rarer and its density decreases.
This is because buoyant force depends on the weight of the fluid displaced by the object submersed in the fluid. The weight of the fluid will be dependent on its volume and density.
Since density of air (fluid) decreases as we go up in altitude, the resultant buoyant force will also decrease.