Think about the DNA coding sequence of a gene. If an A were swapped for a T, what kind of mutation could it cause and why?
a. It could cause a frameshift nonsense or frameshift missense mutation because it would change the reading frame of the codon triplet.
b. It could cause a nonsense mutation because the sequence would no longer be the same, so the protein would be shorter and non-functional.
c. It could cause a silent mutation because A and T are complementary to each other so it is not really a substitution mutation.
d. It could cause a silent, missense, or nonsense mutation because those are the types that can be caused by a nucleotide-pair substitution like this one.

The given situation in the question where an Adenine nucleotide is swapped for a Thymine nucleotide, which as stated are complimentary of each other just like guanine and cytosine, can cause all three types of mutations, frameshift nonsense, frameshift missense and silent because the situation is not specific to either of them. There is no information whether the change resulted in a change in the amino acid structures or not. In this case the best answer is D.

I hope this answer helps.