Hess's law is based on what principle? a) conservation of mass b) conservation of energy c) conservation of entropy d) conservation of neutrality

Answer is B, (Conservation of energy).

According to Hesss Law," For a chemical reaction the enthalpy change is independent of the path followed".

Below figure shows,
 
                                      ΔHf = ΔH₁ + ΔH₂ + ΔH₃

If the water vapor content stays the same and the temperature drops, then the relative humidity increases. its vise versa.if the water vapor content stats the same and the temperature rises, the relative humidity decreases. so, yes, i'd say that temperature and humidity have a strong relationship.

Yes it would be a physical change.