Final answer:
The correct answer to the question is 'operon; prokaryotes'. Operons are clusters of genes functioning together, including a promoter and operator, for the coordinated expression of genes in prokaryotes, such as bacteria.
Step-by-step explanation:
An operon is a cluster of genes that perform related functions found in prokaryotes. Operons allow for the coordinated regulation of a group of genes that are involved in similar processes. Within an operon, you'll find a promoter, operator, and one or more structural genes that are transcribed together. A common example is the lac operon, which includes genes necessary for lactose metabolism in bacteria like E. coli. Unlike prokaryotic operons, eukaryotic genes are composed of exons and introns, with introns being non-coding sequences removed during mRNA processing. Therefore, the correct answer to the question is, 'A(n) operon is a cluster of genes that perform related functions found in prokaryotes' which corresponds to answer choice (d).