Which of the following is true about exons?

Exons are segments of a gene that are retained in the final mRNA after the process of RNA splicing. During transcription, both exons and introns are initially included in the precursor mRNA. However, introns, which are non-coding regions, are removed during splicing. This leaves the exons, which are the coding sequences, to be joined together and ultimately translated into proteins.

Thus, statement B is accurate because it highlights the role of exons as the portions of mRNA that remain after splicing and are involved in coding for proteins. Understanding the distinction between exons and introns is crucial as it underlines the process of gene expression and the functionality of mRNA in protein synthesis.