In a comparison of birds and mammals, the condition of having four limbs is
a. a shared ancestral character.
b. a shared derived character.
c. a character useful for distinguishing birds from mammals.
d. an example of analogy rather than homology

a. A shared ancestral character.

Explanation:

A shared ancestral character is the character or feature that two lineages of same or distinct species have in common derived from a common ancester. The four limbs in birds and mammals is the example of this.

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