A student was given an unknown solution in a test-tube. When he added universal indicator to the test-tube, the solution turned violet. The unknown solution is most likely to be:(1) baking soda solution      (2) washing soda solution(3) caustic soda solution      (4) household ammonia solution

Correct answer: (3) caustic soda solution

Explanation: Universal indicator is used to check the pH of unknown solutions. It turns violet in presence of a strong base, as caustic soda is the strongest in all of the given options, hence it is the correct answer.