I found this image on the internet related to this question.
The correct diagram would be diagram B.
The Christmas tree shape diagram indicates the transcription of DNA to RNA on an operon. The thinner part is where the transcription initiates and the larger part is where the transcription is the most advanced in its process. Many times an operon won't just work for one protein at a time, but will work rather for the transcription of two or more proteins (polycistronic operon). In this case, what we could see on the electron microscope would be not only one Christmas tree (one protein RNA being transcripted) but two or more, one after the other as they all would work within the same operon.