Which statement about jet route altitude ranges is true?

Which statement about jet route altitude ranges is true?

• A. They start at 18,000 MSL and go to FL450
• B. They start at 10,000 MSL and go to FL180
• C. They start at 20,000 MSL and go to FL600
• D. They start at 0 MSL and go to FL200

Answer: (A)

Jet routes are high-altitude en route airways used by faster, jet-powered aircraft. They’re defined to be used from a floor of 18,000 feet MSL up to a ceiling of flight level 450. This 18,000 feet floor marks the boundary between low/medium-altitude routes and the high-altitude jet routes, so the standard description is that jet routes run from FL180 (18,000 feet) up to FL450.

So the statement that jet routes start at 18,000 MSL and go to FL450 matches how these routes are defined and used in practice. The other ranges don’t fit because they either begin below 18,000 feet, end above the typical jet-route ceiling, or extend all the way to the surface, which is not how jet routes are designated.