A difficult problem with cobweb spiders indoors is:

The issue with cobweb spiders indoors that makes choice B the correct answer relates to their ability to create an unsightly mess. Cobweb spiders tend to leave behind feces that can discolor surfaces, especially painted wood, which is often found in homes. This staining can detract from the aesthetics of the environment and may require cleaning or repair efforts, making it a significant concern for homeowners.

While it is true that cobweb spiders can attract other pests, reproduce rapidly, or may be difficult to identify, these factors don't particularly highlight the immediate concern associated with their presence indoors like their feces do. The discoloration caused by their droppings directly impacts the upkeep of the living space, clarifying why this is viewed as a challenging problem related to cobweb spiders in indoor settings.