Final answer:
The genetic code is read in increments of three nucleotides called codons, which specify each amino acid. Experimental evidence confirmed this hypothesis by observing protein synthesis after inserting one, two, or three nucleotides into a gene of a virus. Therefore, the correct option is a. three.
Step-by-step explanation:
The genetic code is read in increments of three nucleotides. Codons, which are groups of three nucleotides, are responsible for specifying each amino acid.
The hypothesis proposed by Francis Crick and Sydney Brenner was confirmed experimentally by inserting one, two, or three nucleotides into a gene of a virus.
It was observed that when one or two nucleotides were inserted, the protein was not produced, but when three nucleotides were inserted, the protein was synthesized and functional. Therefore, the correct option is a. three.