The only living genus of the class Sphenopsida (horsetails) is:

Equisetum, commonly called horsetail or scouring rush, is the only living genus of the class Sphenopsida (also called Equisetopsida). It belongs to the pteridophytes, a group of seedless vascular plants that also includes ferns and club mosses.

Pteridophytes are divided into four classes:

• Psilopsida: the most primitive, such as Psilotum.
• Lycopsida: the club mosses, such as Selaginella and Lycopodium.
• Sphenopsida: the horsetails, such as Equisetum.
• Pteropsida: the ferns, such as Dryopteris, Pteris, Adiantum and Marsilea.

Equisetum has a hollow, jointed stem with clear nodes and internodes, which looks like bamboo. Small, scale-like leaves grow in whorls at the nodes, and branches grow in whorls too, giving some species a brush-like look similar to a horse's tail. The stems are photosynthetic. Their outer walls contain silica, which makes them rough. In the past, people used Equisetum to scrub pots and pans, which is why it is called "scouring rush". Spores are produced in cone-like structures called strobili at the tips of some stems.

Equisetum is often called a "living fossil". In the Carboniferous period, about 300 million years ago, its relatives included giant tree-like horsetails (such as Calamites) that grew more than 15 metres tall. Their remains contributed to the formation of coal deposits.

Like other pteridophytes, Equisetum has a dominant sporophyte (the main plant body), and its male gametes need water to swim to the egg for fertilisation.

Why not the other options?

• ✗Selaginella and Lycopodium belong to Lycopsida (club mosses).
• ✗Marsilea is a water fern belonging to Pteropsida.