You compare homologous nucleotide sequences among several pairs of species with known divergence times. A pair of species that diverged 1 million years ago has two nucleotide differences, a pair that diverged 2 million years ago has four nucleotide differences, and a pair that diverged 3 million years ago has six nucleotide differences. You have sequence data for another pair of species for which the divergence time is unknown. There are five nucleotide differences between them. Based on your clock, when did their line of ancestry diverge? A. 3.5 million years ago
B. 2 million years ago
C. 3 million years ago
D. 2.5 million years ago

Option D, 2.5 million years ago

Explanation:

Given

Pair of species that diverged 2 million years ago has four nucleotide differences

Pair of species that diverged 3 million years ago has six nucleotide differences

A pair of species X has diverged Y million years ago and has five nucleotide differences

These facts clearly indicate that the pair of species X has diverged between the 2 and 3 million years ago as 2 million year ago there was four nucleotide differences and 3 million years ago there was six nucleotide differences. Hence, 5  nucleotide differences would occur in pair of species that diverged between 2 and 3 million years ago.

Thus, option D is correct

option c is the correct answer.