The correct answer is A.
Substitutiopn mutations are single nucleotide mutations in which one nucleotide is substituted by another nucleotide. This can cause different consequences.
In missense substitutions, the nucleotide substitution causes a change in the DNA codon and consequently, the change in the amino acid in the protein.
Nonsense substitutions change the codon for an amino acid into a stop codon that stops the forming of the polypeptide chain prematurely.
Silent substitutions have no evident consequence for the protein, since the change of the nucleotide doesn't change the amino acid. This is possible because there are multiple codons for one amino acid.