William Taft and Theodore Roosevelt both began as Republicans. Why did Roosevelt leave the Republican party?

They supported each other politically but strongly disliked each other personally.

They ran against each other in the General election

Taft defeated Roosevelt in Republican primaries after Roosevelt had already served two terms as a Republican President.

Taft left the Republican to begin the Bull Moose party after being defeated by Roosevelt.

Roosevelt leave the Republican party because they ran against each other in the General election.

Option b

Explanation:

As known and told widely, Taft and Roosevelt begun as republicans and had little political dislike among each other. However, in 1912, Theodore Roosevelt wanted to be elected for president in an individual term through a third political party, The Progressive Party.

He believed in his status among the commoners and others. He believed firmly that, it would be the very way for presidential position for him. But Taft had defeated him in the nomination; yet he stood up against him that year as a third party candidate but he lost to Woodrow Wilson.