C

Given the relationships among the lengths of the emitter ($l_1$), base ($l_2$), and collector ($l_3$) in a transistor, we have the following cases:
1. If all three lengths are equal ($l_1 = l_2 = l_3$), it indicates a condition where the transistor operates symmetrically.
2. If the lengths satisfy the relation $l_3 < l_2 < l_1$, this shows that the collector is the shortest and the emitter is longest.
3. The most suitable answer that indicates a practical configuration of a transistor, where typically the base ($l_2$) is longer than the collector ($l_3$), results in option C: $l_3 < l_1 < l_2$.
Therefore, Option C is the correct answer.