Final answer:
The highest safe standing level on an extension ladder is the fourth rung from the top, according to safety guidelines to prevent falls.
Step-by-step explanation:
The Highest Safe standing level on an extension ladder is the fourth rung from the top. This safety guideline helps prevent overreaching and unbalancing, which could lead to falls and serious injuries. According to the Occupational Safety and Health Administration (OSHA), standing above the fourth rung is not recommended as the ladder becomes unstable and the risk of a fall increases significantly.