The Hartig net, a distinctive structure found in soil, is formed by which type of symbiotic association?

✅Explanation:
Ectomycorrhizae are fungi that form a symbiotic association with the roots of many plants, particularly trees. The Hartig net is the dense network of fungal hyphae that forms between the cells of the root cortex in ectomycorrhizal associations.

📌Other Options Explanations:
-(a) Rhizobium: Rhizobium is a genus of bacteria that forms symbiotic relationships with legumes, fixing nitrogen in root nodules.
-(c) Endomycorrhizae: Endomycorrhizae are fungi that penetrate the cells of plant roots to form structures like arbuscules.
-(d) Protozoa: Single-celled organisms, some can be soil predators.
Important Points
-Mycorrhizae is a symbiotic association between a fungus and roots of higher plants.
-Mycorrhizal fungi allow plants to draw more nutrients and water from the soil.
-They also increase plant tolerance to different environmental stresses.
-They play a major role in the soil aggregation process and stimulate microbial activity. 
-Mycorrhizae play a vital atmospheric nitrogen fixation

• Mycorrhiza is of two types:
-Ectomycorrhiza – In this, the fungus forms a covering around the plant's roots. In this way, a layer like structure is formed on the surface of the roots, which is called peudoparenchymatous sheath. Ex- Pinus, Abies, Eucalyptus and Oak
-They are present in the form of well-developed mycelium on the outside of the root: This is a characteristic feature of ectomycorrhizae. The fungal mycelium forms a sheath around the root, which is a key distinguishing factor from other types of mycorrhizae.
-They increase the area of interface between plant roots and soil: This is also true. The network of fungal hyphae extends into the soil, increasing the surface area for water and nutrient absorption.
-They are found in the cortex of roots of higher plants: This statement is incorrect. Unlike endomycorrhizae, which penetrate the cortical cells of the plant roots, ectomycorrhizae do not penetrate the root cells. Instead, they form a mantle around the roots and extend into the intercellular spaces.
-They absorb and store nitrogen, phosphorus, potassium, and calcium in the fungal mantle: This is another correct characteristic. Ectomycorrhizal fungi play a significant role in nutrient cycling and help in the absorption and storage of essential nutrients, which are then transferred to the host plant.
-Endomycorrhiza – In this the fungus does not form a layer on the roots of plants. They enter intracellularly in the roots. Intracellular fungal hyphae form branched structures in cells of roots called ''arubscule'' and unbranched structures called vesicles so these are also called Vesicular arbuscular mycorrhizal (VAM). Ex – Orchids