In a solution, red litmus turns blue, but blue litmus shows no change. What type of solution is it?

Correct answer: Basic

The governing concept is the colour response of litmus in basic media. A base changes red litmus to blue because hydroxide ions create alkaline conditions. Blue litmus remains blue in the same solution, so the two observations agree. An acid would produce the opposite change, while a neutral solution would change neither strip. Thus, option D, basic, is correct.