Which statement describes exons?

• A. Exons are non-coding segments removed during processing.
• B. Exons are promoter regions.
• C. Exons are a type of RNA polymerase.
• D. Exons are coding regions that are retained and joined together in mature mRNA.

Answer: (D)

Exons are the parts of a gene that stay in the mature messenger RNA after RNA splicing. During processing, introns are removed and the exons are joined in order, forming the continuous sequence that will be translated into protein. This description fits because exons are the portions retained in the final transcript and correspond to the coding information (and any included untranslated regions) that remains to be read during translation. The other statements misplace exons: promoter regions are not part of the transcribed mature mRNA, and exons are not a type of RNA polymerase.