Introduction:
Natural fertilizers are organic substances that provide essential nutrients to plants for their growth and development. They are derived from natural sources such as animal waste, plant materials, and minerals. Among the options provided, cow dung is a natural fertilizer.

Explanation:
Cow dung:
Cow dung is one of the most widely used natural fertilizers. It is rich in organic matter and essential nutrients like nitrogen, phosphorus, and potassium. These nutrients are vital for the healthy growth of plants. Cow dung is often used as a manure or compost, which improves soil fertility and enhances crop yield. It also helps in retaining moisture in the soil and improves its structure.

Neem:
Neem is not a natural fertilizer but rather a natural pesticide. It is commonly used in organic farming to control pests and diseases. Neem extracts or oil can be used to repel harmful insects and protect plants from damage.

Soil:
Soil is not a fertilizer but rather a medium in which plants grow. It provides physical support, water, and nutrients to plants. The quality and fertility of the soil play a crucial role in plant growth. However, to improve soil fertility, natural fertilizers like cow dung can be added.

Earthworm:
Earthworms are beneficial for soil health as they help in the decomposition of organic matter. They consume organic materials like dead plants and animal waste and excrete nutrient-rich castings, also known as vermicompost. Vermicompost is a natural fertilizer that is highly beneficial for plants.

Conclusion:
Among the options provided, cow dung is a natural fertilizer. It is rich in essential nutrients and organic matter, making it an excellent choice for improving soil fertility and promoting healthy plant growth. Neem is not a natural fertilizer but a natural pesticide used to control pests. Soil is a medium for plant growth, and earthworms contribute to soil health by decomposing organic matter and producing vermicompost.