Amongst the options given, the country with the largest livestock population in the world is India (Option 3). India is known for its significant agricultural sector and has a long-standing tradition of livestock rearing. Here are the reasons why India has the largest livestock population:

1. Cultural and Economic Significance: Livestock rearing holds immense cultural and economic significance in India. It is deeply embedded in the country`s rural traditions and plays a crucial role in the livelihoods of millions of people. Livestock, including cattle, buffaloes, goats, sheep, and poultry, are raised for various purposes like milk, meat, eggs, and draught power.

2. Large Rural Population and Agriculture: India has a vast rural population, and agriculture is a dominant sector of the economy. Livestock rearing is often integrated with agricultural practices, allowing farmers to utilize animal waste as organic manure, plough fields with bullocks, and generate additional income from dairy products and animal by-products.

3. Favorable Climate and Geography: India`s diverse geography and climate contribute to the suitability of livestock rearing. The country has various agro-climatic zones, enabling the rearing of different types of livestock. Livestock can thrive in a wide range of environments, from the hot and dry regions in the northwestern parts to the wet and humid regions in the northeastern states.

4. Milk Production: India is the largest milk-producing country in the world, and dairy farming forms a significant part of the livestock sector. The country`s dairy industry is primarily based on indigenous breeds of cattle and buffaloes, which are well adapted to the local climate and have high milk productivity. The availability of milk as a staple food and the demand for dairy products contribute to the large livestock population in India.

5. Sustainable and Small-scale Farming: Livestock rearing in India is predominantly carried out by small-scale farmers, who often practice sustainable and traditional methods of farming. This decentralized approach allows for a distributed livestock population across the country.

It is important to note that while India has the largest livestock population in terms of sheer numbers, other countries may have higher meat or milk production due to different breeding practices or specialized farming systems. However, when considering the overall livestock population, India stands out as the country with the largest numbers.