Answer: D (stainless steel). Surgical instruments must stay hard, be easy to sterilise, and above all resist corrosion. Stainless steel is iron alloyed mainly with chromium (often with some nickel); the chromium forms a thin, protective oxide layer that makes the alloy highly resistant to rusting and corrosion, which is exactly what surgical instruments need.
The other alloys are unsuitable. High carbon steel is hard but brittle and rusts readily, so it would corrode during repeated sterilisation. Low carbon steel and mild steel (which are essentially the same kind of soft, low-carbon iron alloy) are softer and corrode easily, so they cannot be used where corrosion resistance and a tough, sharp edge are essential. None of these three would survive the constant cleaning and steam sterilising that surgical tools must endure.
Only the chromium-rich, corrosion-resistant stainless steel is suitable for surgical instruments, so the answer is D.
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