One way in which Pax Romana and Pax Mongolia are similar is that both were characterized by

a. political stability

b. unifying religious institutions

c. representative forms of government

d. social equality for men and women

The correct answer here would be A.

Political stability would be a characteristic of both Pax Romana and Pax Mongolia. Of course over their long histories both the Mongol and Roman Empire had some political divisions and troubles but on the whole the political situation was stable and it functioned as intended. There weren't that many attempts to overthrow the governments from within, much less those that succeeded.