* Understanding Color in Solutions: Transition metal ions (elements in the d-block of the periodic table) often form colored solutions due to the absorption and emission of light when electrons transition between energy levels.

* The Compounds:

* Al₂(SO₄)₃: Aluminum (Al) is a Group 3 element, not a transition metal. Its ions are typically colorless.

* Cr₂(SO₄)₃: Chromium (Cr) is a transition metal. Chromium(III) ions (Cr³⁺) are known for their green color in solution.

* Fe₂(SO₄)₃: Iron (Fe) is a transition metal. Iron(III) ions (Fe³⁺) can form yellow to reddish-brown solutions.

* Co₂(SO₄)₃: Cobalt (Co) is a transition metal. Cobalt(II) ions (Co²⁺) are typically pink or blue in solution, and Cobalt(III) ions (Co³⁺) can be various colors depending on the ligand coordination.

Conclusion:

Therefore, the compound that is colorless in a water solution is Al₂(SO₄)₃.