Han emperors were so successful at maintaining power because:

The Silk Road established steady trade.
Peasants were regularly pacified and happy.
Succesive emperors proved to be strong leaders.
They oversaw important inventions of paper, wheelbarrows and the like.

I don't know if you still need this answer, but i think it is Successive emperors proved to be strong leaders. Please correct me if im wrong. Hope this helps!

Explanation:

c) troops march forward in close formation against newer, more accurate weapons

false the first governing document in the us history was the articles of federation