Which of the following animals is kept mainly for animal traction?

The animal that is mainly kept for animal traction is the "bullock". A bullock is a castrated male cow that is trained to pull heavy loads, such as carts or plows, as a form of animal power. Bullocks are preferred for animal traction due to their strength and docile nature, which makes them easier to train than other animals such as horses or donkeys. In many parts of the world, bullocks are still used for agriculture and transportation, especially in rural areas where machinery is not readily available or affordable. Therefore, the correct option is "bullock".