Final answer:
The deletion of the Srb4 subunit in the Head module of Mediator in yeast leads to a nearly complete loss of activated transcription.
Step-by-step explanation:
The subunit of the Head module of Mediator that, when deleted, causes a nearly complete loss (~93%) of activated transcription in yeast is Srb4. Mediator is a multi-protein complex that transmits signals from transcription factors to RNA polymerase II; thus, it is essential for the transcription of most protein-coding genes. The Srb4 subunit is critical for the function of this complex, and its absence can severely impair the transcription process in the simple eukaryote model organism, yeast.