The correct answer is: A. Iron
Sandy soil is a type of soil that is high in sand content. Sand is made up of small, loose particles of rock that have been broken down by wind and water. Sandy soil is often light and dry, and it does not hold water well. This can make it difficult for plants to grow in sandy soil.
Iron is a mineral that is found in all types of soil, but it is more abundant in sandy soil. Iron is an essential nutrient for plants, and it helps them to produce chlorophyll. Chlorophyll is the green pigment that plants use to capture sunlight.
Magnesia, potash, and kaolin are all minerals that are found in soil, but they are not as abundant in sandy soil as iron. Magnesia is a mineral that is used to make fertilizer. Potash is a mineral that is used to make soap and glass. Kaolin is a mineral that is used to make paper and ceramics.
In conclusion, the correct answer is: A. Iron. Sandy soil is high in iron content, which is an essential nutrient for plants.