First of all this question's language is somewhat wrong.. there should be mentioned if they're talking about eukaryotes or prokaryotes.. For prokaryotes it's initiation transcriptional level and for eukaryotes answer would be all of these

Regulation of gene expression refers to the control of when and to what extent a gene is turned on or off. It is a highly complex process that occurs at multiple levels, including transcription, processing/splicing, and translation. Therefore, the correct answer is option 'D' - all of these.

Transcriptional Regulation:
- Transcription is the process by which RNA is synthesized from DNA. It is the first step in gene expression and is tightly regulated to ensure that the right genes are transcribed at the right time and in the right amounts.
- Transcriptional regulation involves the binding of transcription factors to specific regions of DNA called promoters and enhancers. Transcription factors can either activate or repress gene expression, depending on their specific function. They can influence the recruitment of RNA polymerase, the enzyme responsible for transcribing DNA into RNA.
- Various mechanisms such as DNA methylation and histone modifications can also impact transcriptional regulation by altering the accessibility of the DNA to transcription factors and RNA polymerase.

Processing/Splicing Regulation:
- After transcription, the primary RNA transcript undergoes processing, including splicing, to produce a mature messenger RNA (mRNA) molecule. Splicing involves the removal of non-coding regions called introns and the joining together of coding regions called exons.
- Splicing is a critical step in gene expression regulation as it allows for the production of multiple protein isoforms from a single gene. This process is controlled by specific sequences within the RNA, as well as RNA-binding proteins that interact with these sequences.
- Alternative splicing can result in different combinations of exons being included in the final mRNA, leading to the production of different proteins with distinct functions or properties.

Translation Regulation:
- Translation is the process by which the mRNA is used as a template to synthesize a protein. It occurs on ribosomes, and its regulation ensures that the correct proteins are produced in the appropriate amounts.
- Translation regulation can occur at various stages, including initiation, elongation, and termination. It involves the interaction of regulatory factors with specific sequences in the mRNA or ribosome, as well as the availability of necessary molecular components.
- Regulatory factors such as microRNAs (miRNAs) and small interfering RNAs (siRNAs) can bind to mRNA molecules and prevent their translation into proteins. Additionally, the stability of mRNA molecules can be regulated, affecting their availability for translation.

In conclusion, regulation of gene expression occurs at multiple levels, including transcription, processing/splicing, and translation. These processes are tightly controlled and coordinated to ensure the proper functioning of cells and organisms.