The correct answer is: a) Compost
Compost is a mixture of organic materials that have decomposed over time. It is a rich source of nutrients and helps to improve soil structure, drainage, and water retention. Compost can be made from a variety of materials, including food scraps, yard waste, and manure.
Sand is a type of soil that is made up of small, loose particles of rock. It does not have a lot of nutrients and does not help to improve soil structure or water retention. Sand can be used to improve drainage in heavy clay soils.
Gravel is a type of soil that is made up of larger, more compacted particles of rock. It does not have a lot of nutrients and does not help to improve soil structure or water retention. Gravel can be used to improve drainage in heavy clay soils.
Lime is a type of soil amendment that is used to raise the pH of the soil. It can be used to improve the growth of plants that prefer alkaline soils. Lime can also be used to control weeds and pests.
In conclusion, compost is the best soil amendment for improving soil fertility and moisture retention.