Before a magnetic compass is adjusted certain correctors must be checked to ensure that they are free of permanent magnetism. These correctors are the __________.

Correct Answer: Option B — Flinders bar and quadrantal spheres

Flinders bars and quadrantal spheres are soft-iron correctors. Before compass adjustment, they must be checked to ensure they are not permanently magnetized, which would introduce errors. Bowditch (The American Practical Navigator) emphasizes that soft-iron correctors must be free of residual magnetism to ensure accurate compensation for induced magnetic effects.