Which of the following is considered a natural magnet?

A natural magnet is a material that occurs in nature and has magnetic properties without any human intervention. Lodestone is a naturally magnetized piece of the mineral magnetite, which is why it is considered a natural magnet. It can attract small ferromagnetic materials due to its inherent magnetic properties.

Iron, while it is a common magnetic material and can become magnetized, is not considered a natural magnet itself because it typically requires processing or treatment to exhibit magnetic properties. Magnetite, although it is a source of natural magnetism, the term “natural magnet” specifically refers to lodestone as the naturally occurring magnetic form people historically used for navigation and other purposes. Nichrome, an alloy of nickel and chromium, does not possess natural magnetic properties and is instead used for specific applications such as heating elements.

Thus, the correct choice is lodestone, as it is a naturally occurring magnetic material that has gained historical significance due to its magnetic nature.