Calculation of Freezing Point Depression
The freezing point depression of a solution can be calculated using the formula:

ΔTf = Kf * m

Where:
ΔTf = freezing point depression
Kf = cryoscopic constant (dependent on the solvent)
m = molality of the solution

Finding the Molality of the Solution
Molality (m) is defined as the number of moles of solute per kilogram of solvent. To find the molality of the solution, we need to calculate the number of moles of HX present in the solution and the mass of the solvent (water) in kilograms.

Number of moles of HX = concentration * volume = 0.2 * 1 = 0.2 moles
Mass of solvent = volume of solution * density of water = 1 L * 1000 g/L = 1000 g = 1 kg

Molality (m) = moles of solute / mass of solvent = 0.2 / 1 = 0.2 mol/kg

Finding the Cryoscopic Constant of Water
The cryoscopic constant of water (Kf) is 1.86 °C/m. This value is a constant and depends on the solvent.

Finding the Freezing Point Depression
Using the formula above, we can calculate the freezing point depression as:

ΔTf = Kf * m = 1.86 * 0.2 = 0.372 °C

Finding the Freezing Point of the Solution
The freezing point of pure water is 0 °C. The freezing point depression of the solution is 0.372 °C. Therefore, the freezing point of the solution will be:

Freezing point of solution = 0 °C - 0.372 °C = -0.372 °C

Conclusion
The freezing point of the 0.2 M aqueous solution of HX with a degree of ionization of 0.3 is -0.372 °C.