**Core Concept**
mRNA synthesis involves the transcription of DNA into a complementary RNA molecule, a process catalyzed by the enzyme RNA polymerase. This process requires the unwinding of DNA double helix and the recruitment of transcription factors and RNA polymerase to specific DNA sequences known as promoters.

**Why the Correct Answer is Right**
The correct answer involves the identification of the key enzymes required for the initiation and elongation phases of transcription. RNA polymerase is the primary enzyme responsible for synthesizing the RNA strand by adding nucleotides to the growing chain. Additionally, transcription factors such as TFIID, TFIIB, TFIIF, and TFIIH are required for the recruitment of RNA polymerase to the promoter region and for the initiation of transcription. These enzymes work together to facilitate the unwinding of DNA and the synthesis of a complementary RNA molecule.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while DNA helicase is involved in the unwinding of DNA, it is not directly involved in the synthesis of mRNA.
* **Option C:** This option is incorrect because while the general transcription factors are required for transcription, they are not the primary enzymes responsible for synthesizing the RNA strand.
* **Option D:** This option is incorrect because while DNA polymerase is involved in DNA replication and repair, it is not involved in the synthesis of mRNA.

**Clinical Pearl / High-Yield Fact**
Remember that the transcription process involves the recruitment of multiple transcription factors and the unwinding of DNA, which can be a critical step in regulating gene expression. This is an important concept to understand for exams and in clinical practice, where transcriptional dysregulation can contribute to various diseases.

**Correct Answer: B. RNA polymerase and general transcription factors.**