KI reacts with CuSO₄ solution to produce cuprous iodide (white precipitate) and I₂ (which gives brown colour) Iodine reacts with hypo (Na₂S₂O₃5H₂O) solution. Decolourisation of solution shows the appearance of white precipitate.

The Compound that forms a brown precipitate with copper sulphate solution and turns white on adding hypo solution is KI (Potassium Iodide).

Explanation:
When a compound reacts with copper sulphate solution, it forms a precipitate if the compound contains an anion (negatively charged ion) that can form an insoluble salt with copper. In this case, the compound (X) forms a brown precipitate, indicating the formation of an insoluble salt with copper.

When hypo solution (sodium thiosulphate) is added to the brown precipitate, it reacts with the copper ions in the insoluble salt, causing them to dissolve. This results in the formation of a white precipitate or solution, depending on the concentration of the copper ions.

Reasoning:
To determine the compound (X), we need to analyze the possible anions present in the given options (K2CO3, KI, KBr, and K3PO4) and their reactions with copper sulphate and hypo solution.

- K2CO3 (Potassium Carbonate): When potassium carbonate reacts with copper sulphate, it forms a white precipitate of copper carbonate, not a brown precipitate. Adding hypo solution does not change the color of the precipitate, making K2CO3 unlikely to be the compound (X).

- KBr (Potassium Bromide): When potassium bromide reacts with copper sulphate, it forms a white precipitate of copper bromide, not a brown precipitate. Adding hypo solution does not change the color of the precipitate, making KBr unlikely to be the compound (X).

- K3PO4 (Potassium Phosphate): When potassium phosphate reacts with copper sulphate, it forms a white precipitate of copper phosphate, not a brown precipitate. Adding hypo solution does not change the color of the precipitate, making K3PO4 unlikely to be the compound (X).

- KI (Potassium Iodide): When potassium iodide reacts with copper sulphate, it forms a brown precipitate of copper iodide. Adding hypo solution reacts with copper iodide, dissolving it and forming a colorless solution. This matches the observation of the compound (X) forming a brown precipitate with copper sulphate and turning white upon the addition of hypo solution.

Therefore, the correct compound is KI (Potassium Iodide).