When must a driver use turn signals?

Using turn signals is crucial for communicating a driver's intentions to other road users, helping to enhance safety on the road. The correct choice indicates that a driver must use turn signals when turning or changing lanes. This practice not only alerts other drivers of your intentions but also allows pedestrians and cyclists to anticipate your movements, reducing the possibility of accidents.

The rationale behind requiring turn signals during these actions lies in the need for clear communication among all road users. Effective signaling ensures that everyone on the road can react appropriately to changes in traffic flow, thus maintaining safety and minimizing confusion.

In contrast, the other options suggest limited or incorrect circumstances for using turn signals. For example, signaling only when merging into traffic or only in residential areas does not encompass the full range of situations where clarity is needed. Signaling solely in the presence of a police officer also neglects the broader responsibility drivers have at all times to indicate their movements.