Syou climb a high mountain, the buoyant force exerted on you by the atmosphere decreases remains the same increases

The answer for the completion exercise shown above is the first option: descreases.

 Therefore, the completed sentence is shown below: "...you climb a high mountain, the buoyant force exerted on you by the atmosphere decreases". 

 As you know, the atmosphere is a fluid and when you rise in altitude, the pressure decreases.