Which of the following metals is ferrous?

The correct choice is related to the definition of ferrous metals, which are those that contain iron. 405 stainless steel is classified as a ferritic stainless steel. It contains iron as the primary element in its composition, making it a ferrous metal.

Understanding the characteristics of the other metals listed provides context for why they do not fit the definition of ferrous metals. MP35N, for instance, is a cobalt-chromium alloy known for its exceptional strength and corrosion resistance, but it does not contain iron, which disqualifies it from being classified as ferrous. Nitinol, an alloy of nickel and titanium, is also non-ferrous as it lacks iron in its composition. Similarly, titanium is a non-ferrous metal that is valued for its strength-to-weight ratio and corrosion resistance but does not contain iron.

Thus, 405 stainless steel stands out as the only ferrous metal in the provided choices, directly linking its iron content to its classification.