What is the degree of freedom for a system comprising liquid water equilibrium with its vapour?

Answer: Option B

Given, vapor-liquid equilibrium :
Number of components $$\left(\mathrm{water}\right)=1$$
Number of phases involved $$\left(\mathrm{vapor}-\mathrm{liquid}\right)=2$$
So, degree of freedom :
$$\begin{array}{l}\left(f\right)=C-\varnothing +2\\ 1-2+2=1\end{array}$$