Final answer:
Iron is the correct answer as it's a magnetic, metallic element commonly used for structural purposes including outdoor stair railings, known for its ferromagnetic properties and versatility in construction.
Step-by-step explanation:
The student's question asks which magnetic, metallic element is extensively used for structural purposes, such as outdoor stair railings. The correct answer is Iron (choice a). Iron is not only magnetic but also very versatile as a structural material used widely in buildings, infrastructure, and appliances.
Metallic properties that define iron include its solidity, shiny appearance, and good conductivity of electricity and heat. Furthermore, iron is ferromagnetic, meaning it has strong magnetic properties and can be magnetized or attracted to magnets. This is why materials like iron are commonly used for items like stair railings, which may also benefit from the base material's ability to be fabricated into durable and robust constructions.
The other options, such as aluminum, nickel, and cobalt, also have their uses in various applications. Nickel and cobalt are indeed magnetic like iron but are used less commonly for large scale structural purposes. Aluminum, while not being magnetic, is known for its lightweight and resistance to corrosion, where strength is less of a concern.