Final answer:
The most plausible ancestor of bats would be a tree-dwelling mammal that had elongated fingers with a membrane for gliding, which later evolved for sustained flight, indicative of homologous structures.
Step-by-step explanation:
An ancestor of bats would most plausibly be an animal that resembled a small, tree-dwelling mammal with features adapted for gliding, similar to what we see in some living flying squirrels. This hypothetical ancestral species would have had elongated fingers and a membrane stretching between them, which would have allowed for gliding through the air.
Over time, these membranes might have evolved to support more powerful and sustained flight, leading to the diverse array of bat species we see today. This evolutionary transition would reflect the concept of homologous structures, where bats share common ancestry with other mammals, including those with forelimbs adapted for various uses, such as the horse's foreleg or the human arm.