The correct answer is b) Lime.
Laterite soils are highly weathered soils that are low in nutrients and organic matter. They are also acidic, which can make it difficult for plants to grow. Adding lime to laterite soils can help to improve their fertility by neutralizing the acidity and increasing the availability of nutrients.
Sand, gypsum, and clay are all minerals that can be added to soils to improve their fertility. However, they do not have the same effect on laterite soils as lime. Sand can help to improve drainage and aeration, but it does not increase the availability of nutrients. Gypsum can help to improve the structure of laterite soils, but it does not neutralize the acidity. Clay can help to retain nutrients, but it can also make the soil more difficult to work.
Therefore, the best way to improve the fertility of laterite soils is to add lime. Lime will help to neutralize the acidity, increase the availability of nutrients, and improve the structure of the soil.