Answer: The correct answer is C. Yellow to orange.
When a base is added to a potassium dichromate (K2Cr2O7) solution, the color change occurs due to the reduction of the chromium(VI) ions (Cr2O7 2-) to chromium(III) ions (Cr3+). The original color of the potassium dichromate solution is yellow-orange.
As the base is added, the solution becomes more alkaline, and the chromium(VI) ions undergo reduction to form chromium(III) ions. This reduction reaction results in a color change from yellow to orange.
Therefore, the correct answer is C. Yellow to orange.
Step-by-step explanation: