The binding of is required for transcription to start.
a protein
dna polymerase
rna polymerase
a transcription factor

A transcription factor

Explanation:

In order for transcription to begin in eukaryotes, not only must RNA polymerase be attached to the double strand of DNA, but several transcription factors must also be binded. Without them the RNA polymerase machinery will not start.

In example GTF's (General transcription factors), also known as basal transcriptional factors, are protein factors that binds into the promoter on DNA to activate transcription. The basic transcriptional apparatus that first bind to the promoter is RNA polymerase + GTFs +a multi-protein complex.

i might be wrong but i think its a. let me know if i’m right and have a good rest of your day!

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.