The correct answer is C) Delicate parts should be cleaned with a brush and an approved safety solvent. This is the correct answer because the proper way to clean electrical contacts is to use a soft brush and an approved safety solvent, as opposed to using compressed air or magnetic brushes, which could potentially damage the delicate contact surfaces. Greasing the contacts to increase contact resistance would also be incorrect, as this could lead to issues with electrical conductivity. The other answer options are incorrect because using compressed air (A) could blow metallic dust into delicate areas, and using magnetic brushes (D) could also potentially damage the contacts. Greasing the contacts (B) would not be the appropriate method for cleaning them.