The correct answer is "they are used to assemble amino acids for protein synthesis".
During translation, a messenger RNA (the product of DNA transcription) is decoded into a protein with a specific sequence of amino acids. This decoding happens by reading the mRNA's nucleotides in groups of three. These triplets of nucleotides are called codons.
Codons mainly are associated with amino acids, but there are some codons which signal the beginning and the end of a protein. There are three "stop" codons (end of the protein) and one "start". Codons are specifying the correct sequence of amino acids in the assembling polypeptide.
