The number of degree of freedom for an azeotropic mixture of ethanol and water in vapour-liquid equilibrium, is

Answer: Option B

For an azeotropic mixture since the composition of a constituent remains same in vapour and liquid one of the variable to define the system reduces.
So, $$f=C-\varnothing +2$$
Given number of components $$=2,$$  number of phases $$=2$$
So, degree of freedom $$=1$$