What is the value of ln y (where y = activity co-efficient) for ideal gases?

Answer: Option A

Since activity co-efficient is defined as
$$\begin{array}{rl}& {\gamma }_i=\frac{\text{Fugacity in real solution}}{\text{Fugacity in ideal solution}},\\ & ln{\gamma }_1=\frac{\overline{n{G^E}_l}}{RT}=0\left(\text{for ideal solution}\right),\end{array}$$
Since there will be no excess gibbs free energy for ideal solution.
So, in the question it will be better if replace ideal gas by ideal solution.