Step 1: Connect ozone loss to chlorine.
Stratospheric ozone destruction over Antarctica is driven by chlorine species released from chlorofluorocarbons.
Step 2: Understand the polar stratospheric clouds.
The extreme winter cold forms polar stratospheric clouds whose ice surfaces host reactions that convert harmless chlorine reservoirs into reactive forms.
Step 3: Identify the reservoir compound formed.
On these surfaces chlorine nitrate, $\text{ClONO}_2$, builds up and later releases active chlorine that destroys ozone.
Step 4: Eliminate peroxyacetyl nitrate.
Peroxyacetyl nitrate is a component of photochemical smog at ground level, not stratospheric ozone chemistry.
Step 5: Eliminate acrolein and sulphuryl chloride.
Acrolein is an organic pollutant and sulphuryl chloride plays no role in ozone depletion.
Step 6: Conclude.
The compound X is chlorine nitrate, option 3.
\[ \boxed{\text{Chlorine nitrate } (\text{ClONO}_2)} \]