Sphenopsida, also known as horsetails, is a group of plants that belong to the division of Pteridophytes. They are characterized by their jointed stems, scale-like leaves, and reproductive structures called strobili. Among the given options, only Equisetum belongs to the Sphenopsida group.

Equisetum:
- Equisetum, commonly known as horsetail, is a genus of plants in the family Equisetaceae.
- They are perennial plants that have hollow, jointed stems with whorls of small, scale-like leaves.
- The stems of Equisetum are also known as "culms," and they are usually green and photosynthetic.
- The leaves of Equisetum are reduced to small, brownish scales that surround each joint of the stems.
- Equisetum plants reproduce through spores, which are produced in cone-like structures called strobili.
- The strobili of Equisetum are terminal structures that contain sporangia, where spores are produced.
- The spores of Equisetum are released into the environment and can germinate into new plants under favorable conditions.

Psilotum, Pteris, and Lycopodium:
- Psilotum and Pteris belong to the division of Pteridophytes, but they do not belong to the Sphenopsida group.
- Psilotum, commonly known as whisk ferns, is a primitive group of plants that lack true roots and leaves.
- Pteris, commonly known as brake ferns, is a group of ferns that have true roots, stems, and leaves.
- Lycopodium, commonly known as clubmosses, is a group of plants that belong to the division of Lycophytes. They are not closely related to Sphenopsida.

In conclusion, among the given options, only Equisetum belongs to the Sphenopsida group. Psilotum, Pteris, and Lycopodium belong to different plant groups within the division of Pteridophytes.