The powder keg of europe refers to:
a) the political unrest in the balkans.
b) many nations vying for power in the balkans.
c) nationalistic fervor in the balkans.
d) all of the choices are correct.

D) all of the choices are correct

political unrest = (explosion of the powder keg, as they were used for muskets)
nationalistic fervor = (usage in the military, which gives off a national feeling)
vying for power = (as the powder keg was used in military, it shows strength in the military)

hope this helps

Your answer should be choice b!