This is not included in organic farming

The correct option is chemical fertilizer.

To understand why this is correct, let us break down the core concept of organic farming:
Organic farming is an agricultural method that aims to produce crops in an environmentally friendly and sustainable manner. A key principle of organic farming is the avoidance or minimization of synthetic, artificial, or chemically manufactured inputs, which can disrupt soil health, beneficial organisms, and ecosystems.

Let us analyze each of the given options to see how they fit into this framework:

1. Crop rotation: This is a practice where different types of crops are grown in the same area in sequential seasons. It helps in maintaining soil fertility, controlling pests, and reducing weeds naturally. It is a fundamental practice of organic farming.
2. Chemical fertilizer: These are synthetic, industrially manufactured plant nutrients (such as artificial nitrogen, phosphorus, and potassium compounds). Because organic farming strictly avoids synthetic inputs, chemical fertilizers are excluded from organic farming practices.
3. Green manures: This refers to growing specific plants (often legumes) and then plowing them into the soil to improve its organic matter and nutrient content. This is a natural, organic method to enrich the soil.
4. Compost and farmyard manures: These are decomposed organic materials (like animal dung, crop residues, and kitchen waste) used to fertilize the soil naturally. They are highly encouraged and widely used in organic farming.

Therefore, because chemical fertilizers are synthetic chemical inputs, they are not included in organic farming.