Germany and Austria-Hungary known as Central Powers because of -
a. the war being fought on two fronts.
b. The strength of their combined armies.
c. their alliance before and during the war.
d. their geographic location in the heart of Europe.

d. their geographic location in the heart of Europe.

Explanation:

The other powers were known as Axis powers as they were not centrally located on the  map of Europe while the two allied forces of Germany and Austro Hungary occupied a central position in the war front.

As such they were known as the central powers combined together because of their operations from the geographical center of Europe while their opposing powers had to fight from the fringes essentially.