Final answer:
Iron is the primary constituent of ferrous alloys, which include a wide range of elements like carbon and chromium with varied properties. So the correct answer is A.
Step-by-step explanation:
The primary constituent of ferrous alloys is iron. Ferrous alloys, including steel, are typically composed of iron with varying amounts of other elements such as carbon, chromium, manganese, nickel, molybdenum, and boron. The presence of iron as the main element in ferrous alloys contributes to their desirable properties, such as strength and resistance to corrosion, making them well-suited for construction, machinery, and various other industrial applications.