Overview:
Phosphorus is an essential nutrient for plants and is required for various physiological processes, including energy transfer, growth, and development. However, phosphorus is often present in soil in forms that are not readily available for plant uptake. In order to enhance phosphorus uptake, plants have developed mechanisms to form symbiotic associations with certain microorganisms, which can facilitate the absorption of phosphorus from the soil.

Symbiotic Associations:
Several microorganisms can form symbiotic associations with plants to enhance phosphorus uptake. These include mycorrhizal fungi, nitrogen-fixing bacteria, and cyanobacteria.

Glomus:
Glomus is a genus of mycorrhizal fungi that form associations with the roots of plants. These associations, known as arbuscular mycorrhizal (AM) symbiosis, are the most common and widespread type of mycorrhizal association in plants. In AM symbiosis, the hyphae of the fungi penetrate the root cells of the plant, forming a mutualistic relationship.

Role of Glomus in Phosphorus Uptake:
The hyphae of Glomus fungi have specialized structures called arbuscules, which are involved in the exchange of nutrients between the fungi and the plant. In the case of phosphorus, the Glomus fungi can extract phosphorus from the soil and transport it to the plant through the hyphae. The hyphae also increase the surface area of the root system, facilitating the absorption of phosphorus from the soil.

Other Options:
Rhizobium is a genus of nitrogen-fixing bacteria that form symbiotic associations with leguminous plants. These bacteria are involved in nitrogen fixation, but their role in phosphorus uptake is not significant.

Frankia is a genus of nitrogen-fixing bacteria that form symbiotic associations with certain non-leguminous plants, such as alder trees. Similar to Rhizobium, their role in phosphorus uptake is not significant.

Anabaena is a genus of cyanobacteria that can form symbiotic associations with certain aquatic plants. They are involved in nitrogen fixation, but their role in phosphorus uptake is limited.

Conclusion:
In conclusion, among the given options, Glomus is the microorganism that helps in the absorption of phosphorus from the soil by plants. Through their symbiotic association with plant roots, Glomus fungi facilitate the uptake of phosphorus by extracting it from the soil and transporting it to the plant. This enhances the plant's ability to access phosphorus, an essential nutrient for its growth and development.