The correct answer is D) resign his naval post.
In order for Theodore Roosevelt to be able to volunteer for military service in Cuba, he had to resign his naval post.
Theodore Roosevelt served in the Spanish-American War as a "rough rider," as a member of the 1st United States Volunteer Cavalry. He resigned to his post as an Assitance Secretary of the Navy. The leader of the regiment was Leonard Wood and Roosevelt was Lieutenant Colone.