Which of the following disqualifies a vehicle from being safe for operation?

The answer is that the vehicle being unsafe for operation is indicated when the windshield is too dirty to see through. Visibility is critically important for safe driving, as a clear view of the road and surroundings is necessary to avoid accidents and react to hazards. If the windshield is dirty, it can significantly impair a driver's ability to see properly, leading to potential dangerous situations on the road.

While backup lights not functioning when the vehicle is in reverse might create a situation where a driver is less aware of their surroundings, it does not inherently make the vehicle unsafe for operation. Additionally, having air conditioning repaired or not having ABS (anti-lock braking system) does not disqualify a vehicle from being driven safely, as long as all other essential safety features are functioning properly. The presence or absence of certain features does not necessarily determine the overall safety of the vehicle.