In which scenario can you reverse the dependent and independent variables and still have a meaningful real-world interpretation of slope ?

As the amount of water vapor in the air increases, the rate at which you perspire increases. 
Reversed: As the rate at which perspire increase, the amount of water vapor in the air increases. Hmm .. nope. 

As the amount of fuel consumed by a vehicle increases, the distance traveled by the vehicle increases. 
Reversed: As the distance traveled by the vehicle increase, the amount of fuel consumed by a vehicle increases. Sounds right. 

As the amount of fertilizer used on a crop increases, the yield of that crop increases. 
Reversed: As the yield of the crop increases, the amount of fertilizer used on a crop increases. Err.. no. 

As the amount of rainfall in a region increases, the groundwater level in the region increases. 
Reversed: As the groundwater level in the region increases, the amount of rainfall in a region increases. That's a no.