C

To solve this problem, we need to analyze the relationships between the lengths of the emitter ($l_1$), base ($l_2$), and collector ($l_3$) of a transistor as given in the options.
Step 1: The correct relationship based on the typical transistor configuration states that for optimal performance, the length of the collector should be greater than the length of the base, which in turn should be greater than the length of the emitter. This gives us the relationship:
$$l_3 > l_2 > l_1$$
Step 2: Looking at the options presented:
- Option A suggests equality between all lengths, which is incorrect.
- Option B suggests that all lengths are equal.
- Option C states the most common transistor design relation.
- Option D suggests that the emitter is the longest, which is incorrect.
Since option C follows the relationship $l_3 > l_2 > l_1$, it is therefore the correct answer.