This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
The sigma (σ) subunit of prokaryotic RNA polymerase 
Select and explain. 
A. Binds the antibiotic rifampicin, B. Is inhibited by α-amanitin, C. Specifically recognizes the promoter site, D. Is part of the core enzyme
C: Specifically recognizes the promoter site
Answer: C: Specifically recognizes the promoter site 
Explanation: Sigma (σ) factor recognizes promotor sequences on the DNA template. Different factors recognize a different group of genes.
α-amanitin inhibit RNA polymerase II of eukaryotes.
Rifampicin inhibits β-subunit of RNA polymerase.
Sigma unit is not a part of core enzyme; it is a part of holoenzyme.

RNA polymerase (holoenzyme) = core enzyme (2α, β,( β’) + sigma (σ) subunit