Answer : 

Explanation:

The correct answer is option 'D' which is Codon. A codon is a sequence of three nucleotides (a triplet) that codes for a specific amino acid during protein synthesis. Let's break down the answer in detail:

Cistron:
- A cistron is a segment of DNA that codes for a single polypeptide chain or a functional RNA molecule.
- It is composed of exons and introns.
- While a cistron contains the coding sequences for a gene, it does not specifically code for an amino acid.

Exon:
- An exon is a coding region of a gene that contains the information necessary to produce a functional protein.
- Exons are transcribed into mRNA and ultimately translated into the amino acid sequence of a protein.
- However, an exon itself does not directly code for an individual amino acid.

Intron:
- An intron is a non-coding region of a gene that is transcribed into mRNA but is removed during RNA splicing.
- Introns do not code for amino acids and are instead involved in the regulation of gene expression and alternative splicing.

Codon:
- A codon is a specific sequence of three nucleotides (such as ACG, UGA, etc.) that codes for a particular amino acid.
- There are 64 possible codons, including start and stop codons, which specify the 20 different amino acids used in protein synthesis.
- The genetic code, which is the set of rules by which codons specify amino acids, is nearly universal across all organisms.

Conclusion:
In summary, while a cistron contains coding sequences, exons are the coding regions, and introns are non-coding regions, the specific sequence of three nucleotides called a codon is responsible for coding for individual amino acids during protein synthesis.

Codon

A codon is a sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis. DNA and RNA molecules are written in a language of four nucleotides; meanwhile, the language of proteins includes 20 amino acids. Codons provide the key that allows these two languages to be translated into each other. Each codon corresponds to a single amino acid (or stop signal), and the full set of codons is called the genetic code. The genetic code includes 64 possible permutations, or combinations, of three-letter nucleotide sequences that can be made from the four nucleotides. Of the 64 codons, 61 represent amino acids, and three are stop signals. For example, the codon CAG represents the amino acid glutamine, and TAA is a stop codon. The genetic code is described as degenerate, or redundant, because a single amino acid may be coded for by more than one codon. When codons are read from the nucleotide sequence, they are read in succession and do not overlap with one another.