Transcription in Eukaryotes
In eukaryotic cells, transcription is the process by which genetic information from DNA is transcribed into RNA molecules. This is a crucial step in gene expression, as RNA serves as a template for protein synthesis.

Heterogeneous Nuclear RNA (hn RNA)
Heterogeneous nuclear RNA (hn RNA) refers to the primary transcript of a gene, which undergoes various processing steps to produce mature messenger RNA (mRNA) molecules. hn RNA is synthesized in the nucleus of eukaryotic cells and is characterized by its heterogeneity and large size.

RNA Polymerases
RNA polymerases are the enzymes responsible for catalyzing the synthesis of RNA molecules. Eukaryotes have three distinct RNA polymerases, each with specific functions:

- RNA polymerase I (Pol I): This enzyme is primarily involved in the transcription of ribosomal RNA (rRNA) genes, which are essential components of ribosomes.
- RNA polymerase II (Pol II): Pol II is responsible for transcribing protein-coding genes, producing precursor mRNA (pre-mRNA) molecules that are further processed to generate mature mRNA.
- RNA polymerase III (Pol III): Pol III transcribes a variety of small non-coding RNA molecules, including transfer RNA (tRNA), small nuclear RNA (snRNA), and 5S ribosomal RNA (5S rRNA).

Transcription of hn RNA
The correct answer to the given question is option 'B', which states that hn RNA is transcribed by RNA polymerase II (Pol II). This is because hn RNA represents the primary transcript of protein-coding genes, which are transcribed by Pol II.

Pol II recognizes specific DNA sequences known as promoters located upstream of protein-coding genes. Upon binding to the promoter region, Pol II initiates transcription by unwinding the DNA double helix and synthesizing an RNA molecule complementary to the coding strand of DNA. This process occurs in the nucleus of eukaryotic cells.

Processing of hn RNA
After transcription, hn RNA undergoes several processing steps to produce mature mRNA molecules. These include:

1. Capping: A modified guanine nucleotide is added to the 5' end of the hn RNA molecule, providing stability and facilitating mRNA export from the nucleus.

2. Splicing: Introns, non-coding regions within the hn RNA molecule, are removed through the process of splicing. Exons, the coding regions, are joined together to form a continuous mRNA sequence.

3. Polyadenylation: A poly(A) tail consisting of multiple adenine nucleotides is added to the 3' end of the hn RNA molecule. This modification enhances mRNA stability and translation efficiency.

4. Nuclear export: Mature mRNA molecules are transported from the nucleus to the cytoplasm, where they can associate with ribosomes for protein synthesis.

Overall, the transcription of hn RNA by RNA polymerase II is a crucial step in gene expression in eukaryotic cells. This primary transcript undergoes various processing steps to generate mature mRNA molecules that can be translated into protein.