Final answer:
After the Spanish-American War, the U.S. saw an increase in responsibility due to new possessions, a strengthened navy, increased agricultural production, and a greater role in international affairs.
Step-by-step explanation:
The United States underwent substantial changes following the conclusion of the Spanish-American War. After the war, the United States gained territorial possessions, such as the Philippines, Puerto Rico, Guam, and influence over Cuba.
This acquisition led to increased global power and recognition, as well as a shift in foreign policy to a more active role in international affairs, contrasting with the choice 'b' in the student's options. Claims that the United States industry decreased or that it became one of the least productive countries are inaccurate; in fact, the U.S. Navy was strengthened and American industry continued to grow.
Four ways the United States changed after the Spanish-American War included: a) The responsibility of the United States increased with its added possessions, c) The navy was built up to support its new role as a world power, d) The involvement in agriculture increased to sustain and capitalize on new markets, and e) The country's involvement in international affairs increased, as it now had overseas territories to manage and protect.