A solution gives green colour with universal indicator. When a small amount of acid is added, the colour becomes yellow. What does this show?

Correct answer: The solution became slightly acidic

The governing concept is the colour scale of a universal indicator. Green generally corresponds to a nearly neutral solution around pH 7, while yellow indicates a lower pH and therefore mild acidity. Adding acid increases the hydrogen-ion concentration and shifts the pH below neutral, producing the yellow colour. Hence option B is correct. The solution did not become more basic, remain unchanged, or turn solid, so A, C, and D do not fit the observation.