Which of the following is described as a natural magnet?

Lodestone is the correct answer because it is a naturally occurring mineral that is magnetized, making it the only option in the list that can be considered a natural magnet. Lodestone (a type of magnetite) has the ability to attract iron and generate a magnetic field, thus exemplifying the properties of magnetism found in nature.

In contrast, clothing fibers are synthetic or organic materials and do not possess any magnetic properties. Marble, being a metamorphic rock, is primarily composed of calcite or dolomite and likewise is not magnetic. Quartz, a mineral made up of silicon and oxygen, also does not display magnetic characteristics. Each of these alternatives lacks the inherent magnetization that defines lodestone as a natural magnet.