Final answer:
A substitution mutation occurs when one base in DNA is replaced by another, and it is the correct answer to the student's question. There are three subtypes of substitution: transitions, transversions, and silent mutations. The correct option is d. Substitution.
Step-by-step explanation:
When one base pair in DNA is replaced with a different one, as in the example from AAATCG to AATTCG, this is called a substitution mutation. There are three subtypes of substitution mutations, which are transitions, transversions, and silent mutations. Transitions occur when a purine is replaced with another purine or a pyrimidine is replaced with another pyrimidine. Transversions, however, happen when a purine is replaced with a pyrimidine or vice versa. A silent mutation occurs when the change in nucleotide does not affect the amino acid sequence of the protein produced.
Mutations have various effects on the DNA sequence. They might be benign, harmful, or occasionally beneficial. These alterations can affect genetic diversity and evolution. Some point mutations replace one amino acid with another, potentially altering a protein's function. If a stop codon is produced, it can terminate the protein synthesis prematurely. Conversely, silent mutations may not impact phenotype since they do not change the amino acid sequence due to the redundancy in the genetic code.
In conclusion, the most correct answer is d. Substitution.