The correct answer is option 'D': all of these.

In eukaryotes, the primary transcript is the initial RNA molecule synthesized from a DNA template during transcription. Before it can be used to produce proteins, the primary transcript undergoes several processing steps to become a mature mRNA molecule. These processing steps include the removal of introns, capping at the 5' end, and polyadenylation (tailing) at the 3' end.

Below is a detailed explanation of each of these processing steps:

1. Removal of introns:
- Eukaryotic genes contain both coding regions called exons and non-coding regions called introns.
- The primary transcript is transcribed as a precursor mRNA (pre-mRNA) molecule that contains both exons and introns.
- The process of removing introns from the pre-mRNA molecule is called splicing.
- Splicing is carried out by a large complex of proteins and small nuclear ribonucleoproteins (snRNPs) called the spliceosome.
- The spliceosome recognizes specific sequences at the boundaries of introns and removes them, joining the adjacent exons to produce a mature mRNA molecule.

2. Capping at the 5' end:
- The 5' cap is a modified guanine nucleotide added to the 5' end of the pre-mRNA molecule.
- The addition of the 5' cap is important for various processes, including mRNA stability, mRNA export from the nucleus, and translation initiation.
- The 5' cap is added soon after the initiation of transcription and involves the enzymatic addition of a methylated guanine nucleotide to the 5' end of the pre-mRNA molecule.

3. Polyadenylation (tailing) at the 3' end:
- Polyadenylation is the process of adding a poly-A tail to the 3' end of the pre-mRNA molecule.
- The poly-A tail consists of a string of adenine nucleotides.
- The addition of the poly-A tail is important for mRNA stability, protection against degradation, and translation termination.
- Polyadenylation occurs after the cleavage of the pre-mRNA molecule at a specific sequence, followed by the addition of multiple adenine nucleotides by the enzyme poly(A) polymerase.

In summary, the processing of the primary transcript in eukaryotes involves the removal of introns, capping at the 5' end, and polyadenylation at the 3' end. These processing steps are essential for the generation of mature mRNA molecules that can be efficiently translated into proteins.