Hypothesized that a structure in birds is homologous to mammals. What best supports this view

A biologist has hypothesized that a structure in the digestive tract of birds is homologous to a structure in the digestive tract of certain mammals. Which of the following findings would provide the best evidence in favor of this view?
A. The genes coding for the two structures utilize the same nitrogenous bases
B. The two structures appear almost identical at an early stage of embryonic development
C. The structures are vestigial in the digestive tracts of both birds and mammals
D. The structures perform similar digestive functions in birds and mammals

The answer key states that B is the right answer, but I do not understand why. Everything that I have read would point to A. Genetic tools being the most effective.

You're misunderstanding what A says.
All DNA in all organisms, plants or animals of any kind, use the same four nucleotide bases to code for protein synthesis. That's why A is wrong. An earthworm's digestive tract is coded using the same nucleotide bases as a human's..it's the order of the bases and the number and types of genes that make the difference, not the molecules used in the code.
B is correct because in very early stages of embryonic development, it is extremely difficult to tell human from bird from any other animal. All early embryos look very, very similar, indicating similar evolutionary orgins.
You're misunderstanding the concept of genetic tools. All living organisms, plants and animals, use the same nucleotide bases to code for their genetic makeup. This does not mean that this indicates any similarities between mammalian and bird digestive systems; it means that all living things use DNA to code, using the same 4 molecules, for all of their traits.
Don't mistake 'using the same nitrogenous bases' for 'having the nitrogenous bases in the same number and order in the DNA'. These are two entirely different statements.