Step 1: Understand bleaching action.
Bleaching agents remove colour by oxidation or reduction of coloured substances.
Step 2: Nature of chlorine.
Chlorine is a strong oxidising agent and bleaches by oxidation in the presence of moisture.
Step 3: Eliminate other options.
\(\text{SO}_2\) bleaches by reduction, while \(\text{HI}\) and \(\text{PH}_3\) are reducing agents.
Step 4: Conclusion.
Thus, chlorine is a powerful bleaching and oxidising agent.