Fun question!
C, but mostly due to A, and rarely if ever B.
I have a hard time remembering ever playing a game out to full OOB victory condition (short of playing through by myself to prove a point.) The most common is a surrender precipitated by a defeat of a large stack or strategic position, which itself is typically preceeded by an ever widening margin of collective IPC / economic gap.
The only time I’ve seen the economic gap be the direct (or at least strongly contributing) cause of surrender is when some large jump in IPC due to a national objective being soundly secured or a smaller power being taken out (ANZAC or Italy) but I’m sure one could argue that being a military victory as well.
Interested to hear other’s thoughts as well, as the three options are somewhat intertwined.