One solution turns red litmus blue and another turns blue litmus red. What will be the main products when they are mixed?

Correct answer: Salt and water

The governing concept is acid-base identification followed by neutralisation. A solution that turns red litmus blue is basic, while one that turns blue litmus red is acidic. When suitable amounts of an acid and a base react, hydrogen ions combine with hydroxide ions to form water, and the remaining ions form a salt: H+ + OH− → H2O. Thus option A is correct; the other products are unsupported.