Introduction:
Potassium dichromate is a chemical compound with the formula K2Cr2O7. It is an orange-colored solid that is highly soluble in water. When dissolved in water, it forms a solution that is initially orange in color.

Explanation:
The change in color of the potassium dichromate solution from orange to yellow is due to a change in the pH of the solution. The pH of a solution is a measure of its acidity or alkalinity. It ranges from 0 to 14, with 7 being neutral, less than 7 being acidic, and more than 7 being alkaline.

pH less than 7:
When the pH of the potassium dichromate solution is less than 7, it indicates that the solution is acidic. In acidic conditions, the orange color of the potassium dichromate solution changes to yellow. This is because the dichromate ion (Cr2O7^2-) present in the solution is reduced to the chromate ion (CrO4^2-), which is yellow in color.

pH 7:
When the pH of the potassium dichromate solution is exactly 7, it indicates that the solution is neutral. In neutral conditions, the orange color of the potassium dichromate solution remains unchanged.

pH more than 7:
When the pH of the potassium dichromate solution is more than 7, it indicates that the solution is alkaline. In alkaline conditions, the orange color of the potassium dichromate solution also remains unchanged.

Unpredictable:
The correct answer is (D) which is unpredictable. This is because the change in color from orange to yellow is dependent on the concentration of the potassium dichromate solution and other factors such as temperature and presence of other chemicals. Therefore, the exact pH at which the color change occurs cannot be predicted accurately.

Conclusion:
The orange color of potassium dichromate solution changes to yellow at a pH less than 7. However, the change in color is unpredictable due to various factors.

Less than 7