Middle-latitude cyclones are fueled by
a. energy from falling precipitation.
b. polar continental air masses.
c. air high up in the atmosphere.
d. the movement of stationary fronts.

I am pretty sure that middle-latitude cyclones are fueled by air high up in the atmosphere because such low-pressure areas usually occur in middle latitudes of the Earth and that's it.  This answer will definitely help you. Regards!

Size and shape of continents