What must be allowed to open and close on the driver side?

The driver side window must be allowed to open and close to facilitate hand signals. This is an important safety feature, as hand signals are often used by drivers to communicate with others on the road, especially in situations where turn signals or brake lights may not be functioning. The ability to open the driver side window enhances visibility and makes it easier for the driver to communicate their intentions, ensuring better safety for themselves and other road users.

Other options, while they may involve features of a vehicle, do not pertain specifically to the requirements for driver communication and visibility in the same critical way. The passenger door, rear window, and sunroof might have their operational importance, but they do not directly support the necessity for hand signaling, which is vital in active driving situations.