Damage to the radial nerve in the spiral groove spares which muscle -
The radial nerve while passing through the spiral groove gives away branches to lateral head and medial head of triceps. The medial head also receives fibers from portion of radial nerve that lies before the spiral groove, through medial muscular branch. Since this is a single answer type of question, the best answer would be long head of triceps.
Lateral head of triceps
Long head of triceps
Medial head of triceps
Anconeus