What might indicate that a driver is about to turn left in front of a motorcyclist?

Answer: B — The driver using their turn signal

A driver using their turn signal is a clear indication that they intend to turn left. Turn signals are designed to communicate a driver's intentions to other road users, including motorcyclists. When a driver activates their left turn signal, it alerts surrounding vehicles and riders that they may cross the path of oncoming traffic, which is crucial information for motorcyclists who need to anticipate potential hazards. In this context, while changes in speed or a driver looking straight ahead could potentially indicate a future action, they are less reliable indicators compared to the explicit signaling of intent provided by a turn signal. Therefore, the clarity and specificity of a driver using their turn signal makes it the most definitive indication that the driver may be about to turn left in front of a motorcyclist.