Which observation suggests that a base may be present in a solution?

Correct answer: Red litmus turns blue

Litmus is an acid-base indicator. A basic solution changes red litmus paper to blue because the indicator responds to the alkaline medium. Blue litmus turning red instead indicates an acid. Limewater becoming milky is associated with carbon dioxide reacting with calcium hydroxide, while a pop sound from a burning splint tests for hydrogen gas. Thus only option A is evidence of a possible base.