What can cause an alternator not to charge the battery?

An alternator not charging the battery can indeed be caused by a blown fuse. Fuses are critical components in the automotive electrical system that protect circuits from overload or short-circuits. If a fuse related to the alternator's charging system blows, it will interrupt the flow of electricity, preventing the alternator from properly charging the battery.

In this scenario, the fuse functions as a safeguard; when it blows, it cuts off the connection necessary for the alternator to send current back to the battery. Therefore, a blown fuse is a plausible and common reason for an alternator failure to charge.

The other options presented do not directly relate to the operation of the alternator or battery charging system. Worn tires, low oil levels, and a dirty engine air filter have no impact on the electrical systems within the vehicle and would not affect the alternator's ability to charge the battery.