The gas in C is the one which is causing the heigh difference between A and B. Coming from C, the pressure has an effect on B. In B the water is pressed and thus the water increases up to A.
It is p = d*g*h, where d stands for density of water and h for the named heigh difference between A and B.
As the density is given in meters and the heigh difference in centimeters, you have one of the to convert. After that you are able to calculate the pressure.