Which process is most likely responsible for the changes that have led to increased size strength and speed in horses?

The process that is most likely responsible for the changes that have led to increased size, strength, and speed in horses is artificial selection. Artificial selection is the process by which humans breed animals and plants for specific traits that they desire. In the case of horses, humans have been breeding them for thousands of years to be faster, stronger, and larger for various purposes such as racing, transportation, and warfare. Through selective breeding, humans have been able to pass on desired traits from one generation to the next, resulting in the development of different horse breeds with specific characteristics. Over time, this process of artificial selection has led to the increased size, strength, and speed of horses.