Answer:
The correct answer will be-gradualism
Step-by-step explanation:
Gradualism is a hypothesis and theory which explains the process of evolution. The concept of gradualism shows that the variation takes place in population in gradual manner. The theory states that a change in the population is the cumulative of all the changes that has taken slowly but in continuous manner.
In the given question, since the evidence which shows the evolution of horses are fossils (old form of life) which showed that the evolution of legs and teeth is slow and gradual which resulted in the phenotype of modern horse.
Thus, gradualism is the correct answer.