A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish-orange. Which of the following would change the colour of this paper to greenish-blue?

Sample solutions turn pH paper yellowish-orange which confirms the acidic nature of the sample. To make the colour greenish-blue, we have to add an antacid.
As the pH paper is able to turn greenish blue in the case of weakly basic compounds and antacids contain weak bases like magnesium hydroxide. Baking soda, sodium bicarbonate, and sodium hydrogen carbonate are bases that can turn pH paper from yellow-orange to green while other substances are acidic in nature and are used as antacids.