The best explanation for the presence of vestigial pelvis and femurs in some snake and whale species is

a common ancestor that used these structures to perform one or more specific functions

Explanation:

A vestigial structure is an organ that has no obvious function in an organism, but it has been used by a past ancestor to perform one or more specific functions. Thus, vestigial structures have been retained in the course of evolution, but they lost their ancestral functions. Vestigial structures/organs can be detrimental for the organism because it is required energy to maintain them and they may suffer diseases (e.g., cancer). Some well-known examples of vestigial organs/structures include, among others, the wings of ostriches and cormorants, the human appendix, etc.

what's the question ?

b brown mice / white sand dunes