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The green colour appears due to the formation of Cr3+ ion in the reaction:

1. Cr2O72- + 3 SO32- + 8 H+ → 3 SO42- + 2 Cr3+ + 4 H2O
2. Cr2O72- + 3 SO32- + 8 H+ → 3 SO42- + 2 Cr2+ + 4 H2O
3. Cr2O72- + 3 SO32- + 8 H+ → 3 SO42- + 2 Cr4+ + 4 H2O
4. Cr2O72- + 3 SO32- + 8 H+ → 3 SO42- + 2 Cr5+ + 4 H2O

Correct answer: Cr2O72- + 3 SO32- + 8 H+ → 3 SO42- + 2 Cr3+ + 4 H2O

Solution

The green color is characteristic of Cr3+ ions, which are formed when dichromate (Cr2O72-) is reduced in an acidic medium. The reaction in option A correctly represents this reduction.

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