Which of the following lines indicates that a correction for magnetic variation is unnecessary?

The agonic line is significant in navigation and aviation because it represents the line on the Earth's surface where magnetic north and true north align perfectly, meaning there is no magnetic variation. When aircraft navigate along or across this line, they do not need to make any corrections for magnetic variation. This is crucial for achieving accurate navigation, as catering to magnetic changes is vital when calculating headings and routes. Therefore, when pilots or air traffic controllers reference the agonic line, they can be assured that no additional adjustments are required to convert between magnetic and true headings.