No window may be covered with which of the following?

Windows on vehicles are crucial for visibility and safety. The correct answer relates to the use of reflective materials, which can significantly impair the driver's ability to see outside the vehicle, especially in bright sunlight or during nighttime driving. When reflective materials are applied to windows, they can create glare or reflections that obscure the view, making it dangerous for both the driver and others on the road.

In the context of vehicle safety inspections, regulations often prohibit the use of such materials to ensure that drivers have maximum visibility to observe their surroundings and react appropriately to potential hazards. This is vital for safe driving practices and reduces the likelihood of accidents resulting from impaired visibility.

The other options, while they might also be restricted in certain contexts, do not carry the same level of risk related to visibility as reflective materials do. Tinted materials can be acceptable within certain limits, dark films are often regulated for how much light they block, and opaque materials, while not generally recommended, may not be as fundamentally dangerous as reflective materials in terms of obstructing a driver's view.