If one solution turns red litmus blue and another turns blue litmus red, what is likely to form when they are mixed?

Correct answer: Salt and water

Red litmus turning blue identifies the first solution as basic, while blue litmus turning red identifies the second as acidic. When an acid reacts with a base, hydrogen ions and hydroxide ions combine to form water, and the remaining ions generally form a salt. This is a neutralisation reaction, although the final mixture’s exact pH depends on the amounts used. Therefore A is correct; metal, oxygen, and sugar do not follow from this test.