Ordinary steel rusts because iron reacts readily with oxygen and moisture in the air. To stop this, stainless steel needs an alloying element that reacts with oxygen even faster than iron does, but instead of flaking off like rust, forms a thin, tightly bonded oxide skin that seals the surface and stops further attack, and if that skin gets scratched it should reform on its own as soon as it sees oxygen again. Chromium is exactly this element: once at least around 10.5 to 12 percent chromium is present in the alloy, it forms a passive chromium oxide film on the surface that gives the steel its corrosion resistance and its stainless name. Nickel is added mainly to keep the steel's crystal structure stable and improve toughness and formability, molybdenum is a secondary addition that helps resist pitting in chloride environments like seawater, and carbon is added for hardness and strength, too much of it can even hurt corrosion resistance by tying up chromium as carbides. So the element that is actually responsible for corrosion resistance in stainless steel is chromium, option (D).