b. a gene has undergone a change in the organisms DNA.
DNA ( deoxyribonucleic acid) contains instructions for protein synthesis in the form of codons ( a sequence of three nucleotides), which code for particular amino acid. DNA is converted to mRNA ( through transcription), which is then converted to proteins ( through translation).
As per the question, a mutation in an organism has lacked the ability to make ( incorporate) one amino acid.
This means that there is a change in the codon ( that is change in the gene) in a manner such that now the mutated codon will not code for any amino acid.
