Picture what happens right at the cutting edge of a tool during high-speed machining, where friction against the workpiece and the flowing chip generates intense, localized heat that can push the tip temperature well past 500 to 600 degrees Celsius. A plain hardened tool would simply soften at that temperature because its martensite would over-temper and its carbides would coarsen quickly. High-speed steels avoid this problem because they are alloyed with strong carbide formers such as tungsten, molybdenum and vanadium, which form very fine, thermally stable secondary carbides that resist coarsening even while the tool is glowing hot during a cut. Since the tool has to hold onto its hardness while red hot rather than simply be tough or lightweight, hot hardness is what decides whether a steel is suitable for high-speed cutting. So the correct choice is option (D).