Assuming the blue wire in your thermostat bundle is actually a Y wire, C is likely to be the brown wire that feeds the red wire for the outdoor unit.
This is assuming you have an air conditioner, and that’s not a float switch for a boiler.
Also, the red wire leading to the outdoor unit looks to have a spot where the insulation has rubbed off or split, you should wrap it with electrical tape to prevent a short circuit.
