Red litmus turns blue in a solution. What nature does it show?

Correct answer: Basic

The governing concept is the litmus test for bases. Red litmus turns blue when it contacts a basic or alkaline solution, so the observed colour change identifies the solution as basic. Therefore option B is correct. An acidic solution turns blue litmus red, and a neutral solution generally produces no characteristic litmus change. Being salt-free is unrelated to this indicator result and cannot be concluded from the observation, so A, C, and D are incorrect.