In an experiment, solution A turns blue litmus red and solution B turns red litmus blue. When mixed in suitable amounts, the temperature rises slightly. This proves which reaction?

Correct answer: Neutralisation

The governing concept is acid-base neutralisation. Solution A turns blue litmus red, so it is acidic; solution B turns red litmus blue, so it is basic. When suitable amounts are mixed, hydrogen ions from the acid combine with hydroxide ions from the base to form water, while a salt is produced. The slight temperature rise is consistent with heat released during neutralisation. Therefore option B is correct, not precipitation, evaporation, or sublimation.