What did imperialism, the opium wars, and the boxer rebellion have in common?

They're strongly related to the British empire

Explanation:

The British empire popularized the concept of Imperialism in the 16th century as an effort to accumulate natural resources in order to strengthen their empire.

Opium wars occurred after trades conflict between the British empire and the Chinese government. Since the Chinese government tried to limit British' economic interest in China, Britain decided to increase the sell of illegal opium to china and cut down their economic productivity due to the increase in addicts.

The Boxer rebellion occurred after there's a lot of resentment because of western's influence in China. At that time, British empire led the coalition against the revolutionaries groups from China.

he sent troops to protect the students as they entered the school.

d) iowa i believe is the answer