For $p= K_H x$. Here $K_H$ is:

Henry's Law constant

The correct answer is Option (2) → Henry's Law constant

The relation: p = Kₕ x represents Henry's Law.

It states that: Partial pressure of a gas above a solution is directly proportional to its mole fraction in the solution. Kₕ is the proportionality constant called Henry’s Law constant.

Option 1: Cryoscopic constant - Cryoscopic constant is used in freezing point depression calculations. It is unrelated to gas solubility.

Option 2: Henry's Law constant - Correct. Kₕ represents the proportionality constant in Henry’s Law.

Option 3: Boyle's constant - Boyle’s law relates pressure and volume of gases. Not related to solubility.

Option 4: Rate constant - Used in chemical kinetics. Not applicable here.