ANS:D - understressing.

Fatigue strength of a metal is improved by understressing. Fatigue strength refers to the ability of a material to withstand repeated cyclic loading without failure. Understressing involves subjecting the material to loads below its fatigue limit or endurance limit during cyclic loading. By operating below this limit, the material experiences less damage and deformation, which can help to improve its resistance to fatigue failure over time. Overstressing, on the other hand, can lead to premature fatigue failure by subjecting the material to loads beyond its fatigue limit. Scratching the surface or raising the temperature generally do not improve fatigue strength and may even worsen it depending on the specific circumstances. Therefore, understressing is the method used to improve the fatigue strength of a metal.