Amajor challenge of nationalism is militarism leadership unification competition.

Militarism and leadership are central ideas of nationalism - they are the intruments that many nationalists use to promote nationalistic goals.

Unification, as in: a unification of a country is also a goal of nationalism, especially if the country is split.
So we reject those options and  our answer is "competition"!

Dall of the above all of the choices make sense