Which of the following bats is known to be solitary?

Keen's Bat is recognized for its solitary behavior, which sets it apart from some other bat species that often roost in colonies. This solitary nature can be attributed to its habitat preferences and ecological adaptations. Keen's Bat typically prefers environments where suitable roosting sites are less clustered, leading to a lifestyle that does not depend on the social structures commonly seen in more gregarious species.

In contrast, the Little Brown Bat and Big Brown Bat are known for their tendency to roost in larger groups, especially in places like caves or buildings, which provides advantages for warmth, protection from predators, and increased reproductive success. The Mexican Free-tailed Bat is renowned for its large colonies, often numbering in the millions, particularly in the summer months when they converge in large roosts. This stark difference in social behavior makes Keen's Bat unique among the listed species.