Final answer:
Spain's refusal to grant full Cuban independence was the main reason President McKinley was unable to avoid war with Spain, despite various pressures including strong public opinion and economic interests in Cuba. The correct option is A.
Step-by-step explanation:
President McKinley was unable to avoid a war with Spain primarily due to Spain's refusal to grant Cuba its independence. McKinley had urged Spain to find a peaceful solution and to end its military camps policy, but Spain only agreed to partially meet the demands without guaranteeing Cuban independence.
The ultimate refusal led McKinley to request a declaration of war from Congress, influenced by humanitarian concerns, public opinion favoring war, and economic interests in Cuba. Despite his own reservations, pressure from public opinion and the patriotic fervor of the time made it difficult for McKinley to maintain a stance against intervention.
Additionally, the Teller Amendment passed by Congress, which barred the U.S. from annexing Cuba post-war, aimed to assure those worried about imperialistic intentions.