The correct option is A The relationship between the amount adsorbed by an adsorbent and the pressure (or concentration) of the gas (adsorbate) at a constant temperature.
The relationship between the amount adsorbed by an adsorbent and the pressure (or concentration) of the gas (adsorbate) at a constant temperature is called the adsorption isotherm.
The quantity of gas adsorbed by a given amount of adsorbent depends upon both temperature and pressure. At given temperature, adsorption of a gas on solid surface increases with increase in pressure of the gas.