An agonic line is defined as:

• A. A line on the Earth's surface where magnetic variation is maximum.
• B. An irregular imaginary line where magnetic and geographic poles are aligned and there is no magnetic variation.
• C. A line indicating the equator.
• D. A line marking flight corridors.

Answer: (B)

Magnetic variation is the angle between true north (geographic north) and magnetic north. The agonic line is the imaginary line on the Earth’s surface where that variation is zero, meaning magnetic north and geographic north line up. Along this line, a compass points directly to true north because there’s no declination to offset the reading. This line is irregular and shifts over time as the Earth’s magnetic field changes, so it isn’t a fixed, simple shape. The other ideas aren’t about zero declination: an equator is a latitude line with no relation to magnetic variation, and flight corridors are unrelated navigation routes.