For an ideal solution, the activity coefficient is defined as

(a) \(\gamma_{i}=\frac{f_{i} x_{i}}{\bar{f}_{i}}\)

(b) \(\gamma_{i}=\frac{x_{i}}{a_{i}}\)

(c) \(\gamma_{i}=\frac{a_{i}}{x_{i}}\)

(d) None of these.