What is the number of degrees of freedom for liquid water in equilibrium with a mixture of nitrogen and water vapor?

Answer: Option A

Given, number of components $$=2$$
Number of phases $$=2$$
$$\begin{array}{rl}& \text{So, }f=2-2+2\\ & \Rightarrow f=2\end{array}$$
Assuming no reaction or reaction taking place very slowly.