Final answer:
The safest method to begin backing up a vehicle is to have a spotter guide you, ensuring a clear understanding of the immediate environment and reducing the risk of accidents.
Step-by-step explanation:
The safest way to begin a backing maneuver is C) Have a spotter guide you from outside the vehicle. When you have a spotter, they can give you direct guidance and signals to help you back up safely, reducing the risk of accidents. Using mirrors is not the safest option because they have blind spots. Quickly accelerating can lead to loss of control and is dangerous. Horn honking can inform others of your intentions, but it does not ensure safety as it doesn't give you any information about obstacles around you.