[[1]] turns red litmus to blue.

An indicator is a chemical that, when added to an acidic or basic solution, changes its color up to a certain point in response to the pH of the solution. The majority of the time, indicators are either weak acids or weak bases having the unique property of showing different colors in both the ionization and unionized form, for example, phenolphthalein is a weak acid. An aqueous solution mostly made up of lichens is infused into wood cellulose to create litmus paper. The lichens are allowed to ferment in a mixture of potassium carbonate, ammonia, and a tiny amount of sulfuric or hydrochloric acid during the making of red litmus paper. The base is the one that turns red litmus blue and acid is the one that turns blue litmus red.

A litmus paper changes color according to hydrogen ion concentration and thus is used to test pH. Two types of litmus papers are there; blue and red. An acidic substance can change blue litmus paper to red. A basic substance turns red litmus paper to blue. Therefore, the correct answer is base.