How can other vehicles be warned at night about a vehicle carrying an extension or load greater than 1.5 meters?

• A red flag should be attached to the end of the extension or load.
• A blue flag should be attached to the end of the extension or load.
• A blue light must be attached to the end of the extension or load.
• A red light must be attached to the end of the extension or load.
  Explanation:

  "At night, a red light must be attached to the end of the extension or load."