Which hand signal is appropriate for indicating a left turn while riding a motorcycle?

The appropriate hand signal for indicating a left turn while riding a motorcycle is to extend the left arm straight out to the side. This signal is universally recognized among motorcyclists and drivers as a clear indication that the rider intends to make a left turn. Utilizing the left arm emphasizes visibility to others and adheres to standard signaling practices, enhancing road safety.

The other options do not convey a left turn accurately. Pointing upward is commonly interpreted as a signal to indicate a right turn or a lane change, while pointing downward can be associated with stopping or indicating a desire to pull over. Waving left lacks clarity and could be misunderstood by surrounding traffic. Therefore, extending the left arm straight horizontally is the most effective and widely understood method for communicating a left turn in motorcycle riding.