Final answer:
Transcription begins when RNA polymerase binds to a DNA strand at the promoter site, initiating mRNA synthesis by reading the coding DNA strand and adding complementary ribonucleotides.
Step-by-step explanation:
Transcription proceeds when RNA polymerase binds to a strand of DNA. This is the correct answer because the initiation of transcription begins when RNA polymerase attaches to a region of a gene called the promoter, signaling the DNA to unwind. During this process, it reads the bases in one of the DNA strands and is ready to synthesize a strand of mRNA with a complementary sequence of bases. As transcription progresses, RNA polymerase facilitates the elongation stage, adding the correct ribonucleotides that are complementary to the coding DNA strand, eventually building a strand of mRNA.