Final answer:
D) mRNA is not a constituent of a cell's proteome as it is involved in protein synthesis but is not a direct component.
Step-by-step explanation:
The correct answer is d) An mRNA.
Constituents of a cell's proteome include various types of proteins, such as enzymes, cytoskeletal proteins, and transport proteins in the plasma membrane. These proteins play crucial roles in cellular processes and functions. However, mRNA (messenger RNA) is not considered a constituent of a cell's proteome as it is involved in protein synthesis and serves as a template for the production of proteins by ribosomes but is not a direct component of the proteome itself.