All that matters for C is that there is a closed circuit. So Kurchoves Voltage law means that going around in a circle from any point back to the same point, MUST result in a voltage of 0.
So starting at d (because it is common zero volts) and going clockwise:
14V + (10*I) + (10*I) + 6V = 0 (where I is current through resistor).
0 = 20V + 20*I.
I = -1;
V=IR. So each resistor has -10V across it ( you drop 10V moving from one side of the resistor to the other side when you move clockwise through the circuit).
So if you move from common to point C, in clockwise direction, you go from 0V up to 14V then cross the resistor is -10V, arriving at 4V.
All of this can also be confirmed by going "anti close wise" around the circuit:
-6V + 10*I + 10*I - 14V = 0
So I = 1, So voltage across resistor is +10V in anti clockwise direction (you gain 10V when moving from one side of the resistor to the other side in anti clockwise direction).
So moving from common to C: 0 - 6V + 10V = 4V.
Both ways tell us C is 4V with respect to common 0V.