Which of the following units is useful in relating concentration of solution with its vapour pressure?

We know that,

Henry's law states that at a constant pressure, the solubility of a gas is directly proportional to the partial pressure of the gas.

So, if we take mole fraction of a gas in the solution as a measure of its solubility.

Then,

According to Henry's law, partial pressure of gas in the solution is proportional to the mole fraction of gas in the solution.

$\mathrm{p}={\mathrm{k}}_{\mathrm{H}}\mathrm{x}$

where, ${\mathrm{k}}_{\mathrm{H}}=$ Henry's constant

              $\mathrm{x}=$Mole fraction

Hence, option(a) is the correct choice.