Which of the following might indicate a termite issue?

The presence of hollow-sounding wood and frass is a strong indicator of a termite issue. Termites consume wood from the inside out, leading to the hollow sound when tapping on infested wood, as much of the structural integrity has been compromised. Frass, which are termite droppings, often looks like small pellets and can be found near areas where termites are actively feeding. Together, these signs clearly point to an active termite infestation, making this choice the most definitive answer regarding termite presence.

Other options may suggest issues related to water damage or other pests but do not provide the same direct evidence of termites. For instance, signs of water damage could indicate moisture problems that attract various pests but are not specific to termites. Fresh mud on walls can also be a sign of a termite mud tube, but it may not be as conclusive on its own without additional supporting indicators such as the condition of the wood. Visible insects flying during summer could signify various types of insects, including swarming termites, but it does not definitively confirm an infestation like the specific signs of hollow wood and frass do.