Final answer:
The correct answer to the question is 'C) Site', as transcription factors assemble on a promoter and bind to a start site located 25 to 30 base pairs upstream of the transcription initiation site.
Step-by-step explanation:
The assembly of transcription factors on a promoter begins 25 to 30 base pairs upstream, where it binds to a start site. The nucleotide pair in the DNA double helix that corresponds to the initiation site from which the first 5' RNA nucleotide is transcribed is called the initiation site. Within the promoter region, there is a sequence known as the TATA box, which is a key component in forming the initiation complex for transcription.
The TATA box, containing the consensus sequence 5'-TATAAA-3', is located 25 to 35 bases upstream of the transcriptional start site. Transcription factors bind to the TATA box to recruit additional factors, ultimately allowing RNA polymerase to bind to the promoter. This event sets the stage for the DNA to be transcribed into RNA, and transcription begins at the transcription initiation site, just downstream of the TATA box.