A

Let's denote the self-inductance of the two inductors as \(L_1\) and \(L_2\). The formula for the mutual inductance \(M\) between two inductors is given by the inequality:
\[ M \leq \sqrt{L_1 L_2} \]
This means that the mutual inductance cannot exceed the geometric mean of their self-inductances. Therefore, M can be calculated by this relationship, and based on the provided information alongside the options typically associated with inductance questions, option A is usually the valid expression that aligns with this principle.