The coding segment of DNA is called

A gene is made up of two kinds of DNA segments: exons, which are the coding sequences that are retained in the mature mRNA and are translated into protein, and introns, which are non-coding intervening sequences that are removed during RNA splicing before translation.

The question asks specifically for the coding segment of DNA that is expressed as protein — after the introns are spliced out, the remaining exons are joined together to form the mature mRNA, so this coding segment is called the exon.

• Muton — the smallest unit of DNA capable of undergoing an independent mutation (often a single nucleotide pair); it is a unit of mutability, not a coding-vs-non-coding classification.

• Intron — the non-coding intervening sequence within a gene that is removed during RNA splicing and never appears in the mature mRNA.

• Codon — a triplet of three nucleotides in mRNA that specifies a single amino acid during translation; it is a reading unit found inside an exon, not the name for the coding DNA segment itself.

Hence, exon is the correct term for the coding segment of DNA.