A malfunctioning furnace could lead to symptoms such as which of the following?

A malfunctioning furnace can lead to a variety of health and environmental concerns, particularly regarding indoor air quality. When a furnace operates improperly, it can release pollutants such as carbon monoxide, dust, and other particulates into the air. These pollutants can cause symptoms like headaches and watery eyes, which are indicative of respiratory irritation or allergic reactions. This highlights the importance of maintaining and servicing a furnace regularly to ensure it operates efficiently and safely.

In contrast, the other options do not have a direct correlation with a malfunctioning furnace. Increased lumens in light fixtures are related to electrical issues rather than heating systems. Unusual noises from the refrigerator pertain to appliances operating independently of the heating system. Lastly, intermittent internet connectivity is associated with network issues and does not involve the heating equipment at all. Thus, the connection between a malfunctioning furnace and health symptoms makes the focus on headaches and watery eyes the correct choice.