Final answer:
TFIIH is a transcription initiation factor that helps form the transcription bubble by unwinding DNA to allow mRNA synthesis.
Step-by-step explanation:
TFIIH, responsible for forming the transcription bubble, is best described as a transcription initiation factor. This complex plays a pivotal role during the initiation phase of transcription, where it contributes to the recruitment of RNA polymerase II to a protein coding gene. The transcription initiation factor is involved in unwinding the DNA double helix at the initiation site, allowing RNA polymerase to synthesize mRNA.