What remains in the mature mRNA after introns have been removed?

The correct answer is exons, as these are the segments of the mRNA that remain after the introns have been spliced out. In the process of mRNA maturation, the initial transcript (pre-mRNA) contains both exons, which are the coding regions, and introns, which are non-coding regions. During the splicing process, the introns are removed, allowing the exons to join together. This results in a mature mRNA molecule that contains only the exons, which will then be translated into a protein.

The other options reflect components that are either part of the initial mRNA structure or represent the result of mRNA translation, but they do not accurately describe what remains in the mature mRNA. Codons are sequences of three nucleotides in the exons that specify amino acids but do not represent the entire mRNA. Introns, by definition, are the sequences that are removed and thus do not remain in the mature mRNA. Amino acids are the products of the translation of the mature mRNA, rather than components of the mRNA itself.