The amino acid sequence of a protein is altered due to a mutation in the nucleotide of a nucleic acid. This condition is called nonsynonymous mutation.
“Indel” is a type of mutation, which means insertion or deletion of a nucleotide from a deoxyribonucleic acid (DNA) sequence. In phylogenetic studies, these mutations are used as genetic markers.
A mutation in an exon mostly results in change in the sequence of a protein by altering the addition of an amino acid. Thus, it codes for a nonsynonymous mutation, which includes alteration of an amino acid. Hence, the option (b) is not correct.
An indel mutation can occur either in an intron or an exon. Hence, the options (c) and (d) are not correct.
A mutation in an intron has no effect on the sequence of protein. This is because an intron is a non-coding sequence. Thus, the nonsynonymous mutation is not possible in introns.
Hence, option (a) is the correct answer.