If blue litmus turns red in a solution and bubbles form when carbonate is added, which conclusion is correct?

Correct answer: The solution is acidic

The governing concept is the characteristic reaction of acids. An acid turns blue litmus red, showing that the solution is acidic. Acids also react with carbonates to form a salt, water, and carbon dioxide; the carbon dioxide escapes as visible bubbles. Therefore option A is correct. A basic or neutral solution would not give both of these observations under the stated conditions.