What amendment was added to the War Declaration assuring Cuba’s independence after peace was restored?

The correct amendment is the Teller Amendment. This amendment was attached to a joint resolution declaring war with Spain in 1898, and it explicitly stated that the United States would not establish permanent control over Cuba following the war. The purpose was to assure both Congress and the Cuban people that the U.S. was committed to Cuba's independence, rather than seeking territorial expansion.

The Platt Amendment, on the other hand, was established later, in 1901, and it allowed significant U.S. influence over Cuban affairs, including interventions in Cuban politics and the establishment of Guantanamo Bay as a U.S. naval base. The Open Door Policy refers to the United States' foreign policy aimed at ensuring equal trading rights in China, and the Boxer Protocol was an agreement that ended the Boxer Rebellion in China. These latter two options are unrelated to Cuba's independence and the context of U.S. involvement during the Spanish-American War.