A

In transistor configurations, the current gain \( \beta \) can be defined as the ratio of the collector current \( I_C \) to the base current \( I_B \). When we consider the lengths of the respective limbs (usually in terms of physical layouts of transistors), we need to use the ratio of the lengths corresponding to these currents. In this case, we relate the lengths through the equation: \( \beta = \frac{l_B}{l_C} \). Thus, option (a) is correct.
Therefore, A.