For an ideal liquid solution, which of the following is unity?

Answer: Option C

For an ideal liquid solution the activity coefficient will be equal to 1. Since, activity coefficient of a species is defined as
$${\gamma }_i=\frac{\text{fugacity of the species in the real solution}\left(\widetilde{f_l}\right)}{\text{fugacity of the species in the ideal solution}\left(\widetilde{{f_l}^{ld}}\right)}$$
For ideal solution $$\widetilde{f_l}=\widetilde{{f_l}^{ld}}$$
$$\mathrm{So},{\gamma }_i=1$$