Final answer:
Earth's magnetic field indicates that its outer core is made of liquid metal, such as iron and nickel, which generates electric currents and a magnetic field.
Step-by-step explanation:
The Earth's magnetic field is a phenomenon that results from the flow of liquid metal in the outer core of the planet, which generates electric currents and consequently a magnetic field. As metals such as iron and nickel are magnetic and facilitate this process, while sediment and rocks do not produce magnetic fields, the presence of Earth's magnetic field is a clear indication that part of Earth's interior, specifically the outer core, must be made of metal. Therefore, the correct answer is (b) Made of metal, which allows Earth to behave like a giant bar magnet with a magnetic field aligned with its rotational poles.