Which One of the Following Statements Describes the Process Through

Which one of the following statements describes the process through which a primary RNA transcript becomes a functional mRNA?

A) Sections of the primary transcript called exons are removed, and the sections that remain are spliced together.
B) Sections of RNA called exons are added to the primary transcript.
C) A primary transcript needs no editing and is immediately ready to function as mRNA.
D) Noncoding sections of the primary transcript, called introns, are removed, and the remaining coding sections, called exons, are spliced together.
E) Random sections of the primary transcript called exons are replaced with sections called introns.