A

In a common-emitter transistor configuration, the current gain \( \beta \) is defined as the ratio of the collector current to the base current. Mathematically, it is expressed as:
\[ \beta = \frac{I_c}{I_b} \]
To express the length parameters in terms of \( l_b \) and \( l_c \) used to define the transistor configuration, we can also relate them as \( \beta = \frac{l_b}{l_c} \). Therefore, the correct answer is option (a): \( \frac{l_b}{l_c} \).