Question

two organisms have inherited the same gene from a common ancestor. this gene might have different sequences in the two organisms because ______.
the gene has undergone mutation in one of the organisms but unchanged in the other
they are not related
the gene has undergone mutations over generations in each group of organisms
the two organisms use different genetic codes

Explanation:

The question states the organisms share a common ancestor, so they are related (eliminating "they are not related"). The genetic code is nearly universal, so different codes are unlikely (eliminating "the two organisms use different genetic codes"). While mutation in one organism is possible, the most general reason for sequence differences in related organisms is cumulative mutations over generations in each lineage, not just one. Thus, the correct answer is that the gene mutated over generations in each group.

Answer:

the gene has undergone mutations over generations in each group of organisms