Answer:
The correct answer would be 40 million years.
The molecular clock is a technique used to determine the time when the two species diverged from a common ancestor. It uses the mutation rate to determine the same.
Mutation rate is the rate at which a number of mutations take place in a given unit time.
If we compare the DNA sequence of the given two species, we would find that three mutations have taken place in thirty million years.
Species A: GACTGGGCATTA
Species B: GACCGGACATGA
Thus, the mutation rate of the species is one mutation per 10 million years.
The DNA sequences already have 3 mutations. Thus, for a total of 7 mutations, 4 mutations are required.
Thus, in 40 million years the two DNA sequences would differ by 7 nucleotides.