When downshifting, what is an important time to perform this action?

Answer: B — Before starting downhill

Downshifting involves shifting to a lower gear in order to slow down the vehicle. This is important when going downhill as gravity will cause the vehicle to gain speed and may also cause brake overheating if only brakes are used for slowing down. Downshifting before starting downhill allows the engine to help slow down the vehicle and can prevent potential braking issues. The other options do not involve situations where downshifting is necessary as the vehicle is already slowing down or turning.