Where to connect c wire in furnace
The other white wire is your clue

See that two-wire cable with the red wire connected to the yellow (Y) wire from the thermostat cable? That cable controls your compressor, and the white wire from it must connect to the equivalent of the C terminal on your air handler for your air conditioner to work.  So, connect whatever wire you're going to use as C to the same terminal as that white wire, and you'll be good!