The correct answer is A. 2100.
The coefficient of compressibility is a measure of how much a substance decreases in volume when it is subjected to pressure. It is defined as the reciprocal of the bulk modulus, which is the ratio of the pressure applied to a substance to the change in volume that results.
The bulk modulus of water is 2.2 x 10^9 Pa. The reciprocal of this is 4.5 x 10^-10 m^2/N. To convert this to kg/cm^2, we multiply by 9.80665 N/kg and divide by 100 cm^2/m^2. This gives us 2100 kg/cm^2.
Option B is incorrect because it is the bulk modulus of water, not the coefficient of compressibility. Option C is incorrect because it is the coefficient of compressibility of air, not water. Option D is incorrect because it is the coefficient of compressibility of steel, not water.