Cows and mice have the same amino acid sequence for topoisomerase I, an enzymatic protein involved in unwinding the DNA double helix. The fact that these two different species have this identical amino acid sequence provides evidence that -

They share a common ancestor

Explanation:

Due to having the same Amino acid sequence for Topoisomerase I, it can be inferred that during their evolutionary development, cows and mice had a common ancestor with that same amino acid sequence for Topoisomerase I.

a, i am pretty sure.