Under which of the following circumstances can a flea infestation occur in a vacant home?

A flea infestation can occur in a vacant home primarily due to the presence of an animal in the home or crawl space. Fleas require a host for survival, and if an animal, such as a dog or cat, was once living in the home or has accessed crawl spaces, it can leave behind flea eggs, larvae, and pupae. These life stages can remain dormant for extended periods, waiting for suitable conditions and a host to emerge.

While human presence may contribute to flea populations and certain environmental factors like warm temperatures may facilitate their growth and activity, the critical factor for initiating an infestation in a vacant space is the presence of an animal. Additionally, standing water typically relates more to other pest infestations, like mosquitoes, rather than fleas. Therefore, the correct answer emphasizes the pivotal role that an animal plays in establishing or perpetuating a flea infestation.