The permanent bleaching effect is caused by:

The correct option is Chlorine gas.

To understand why chlorine gas causes a permanent bleaching effect, let us look at the chemical processes behind bleaching by different agents like chlorine (${\mathrm{Cl}}_2$) and sulfur dioxide (${\mathrm{SO}}_2$).

Bleaching can occur either via oxidation or via reduction:
1. Bleaching by Oxidation (Permanent): Chlorine gas bleached materials through oxidation. In the presence of moisture or water, chlorine reacts to release nascent oxygen.
The reaction is:
${\mathrm{Cl}}_2+H_{2}O\to \; 2HCl\; +\; [O]$
The nascent oxygen $\left[O\right]$ then reacts with the colored organic matter to oxidize it into a colorless product:
$\text{Colored substance}+\; [O]\; \to \text{Colorless substance}$
Since the oxidized colored compound cannot easily be reduced back by atmospheric oxygen, this bleaching action is permanent.

2. Bleaching by Reduction (Temporary): On the other hand, sulfur dioxide gas (${\mathrm{SO}}_2$) bleaches materials via reduction by removing oxygen or adding hydrogen. When the bleached colorless product is exposed to air, it gets re-oxidized by atmospheric oxygen back to its original colored form, making the bleaching effect temporary.

Therefore, because chlorine bleaches via oxidation, it results in a permanent bleaching effect.