If a magnetic compass is not affected by any magnetic field other than the Earth's, which statement is TRUE?

Correct Answer: Option C — Compass error and variation are equal.

Compass error is the algebraic sum of variation and deviation. If deviation is zero, the compass error is solely equal to the variation. This is a fundamental principle of navigation, as described in Bowditch, Volume I, regarding the relationship between true, magnetic, and compass headings.