The correct maneuver for reducing speed before taking a left turn is to:

The most effective method for reducing speed before making a left turn involves closing the throttle and braking simultaneously. This approach provides the best control over the motorcycle as it allows the rider to decrease speed and adjust balance without causing the bike to destabilize.

When the throttle is closed, the engine deceleration helps in reducing speed while also ensuring that the weight shifts to the front tires, enhancing traction during the turn. Applying the brakes at the same time aids in further reducing speed while providing additional stopping power. This simultaneous action is critical because it helps maintain stability and control, ensuring that the motorcycle is at an appropriate speed for negotiating the turn safely.

The other options may not offer the same level of control or safety. For instance, applying the rear brake only may lead to instability and lockup of the rear wheel. Shifting gears quickly without the throttle closed may not effectively reduce speed and could unsettle the bike. Letting off the gas and coasting can lead to inadequate speed reduction, which may result in entering a turn too fast. Thus, closing the throttle and braking simultaneously is the most reliable maneuver for a safe and controlled left turn.