The correct answer is: D. Acidic soils.
Gypsum is a mineral that is used to improve the pH of acidic soils. It does this by adding calcium and sulfate ions to the soil, which neutralize the acidity. Gypsum can also help to improve drainage and aeration in acidic soils.
Alkaline soils are soils that have a pH of 7 or higher. These soils are often found in areas with high rainfall or where there is a lot of limestone in the soil. Alkaline soils can be difficult to grow plants in, as they can cause nutrient deficiencies and make it difficult for plants to absorb water.
Saline soils are soils that have a high concentration of salts. These soils can be found in areas with high evaporation rates or where there is a lot of salt in the water table. Saline soils can also be difficult to grow plants in, as the salts can interfere with plant growth.
Podsols are soils that are found in cool, wet climates. They are characterized by a thick layer of organic matter on the surface, followed by a layer of leached clay and sand. Podsols are often acidic and low in nutrients.
Gypsum is not used to improve saline soils or podsols. Saline soils are improved by leaching the salts out of the soil, while podsols are improved by adding lime to the soil.