If blue litmus turns red in a colourless solution, which ion is likely to be in excess in that solution?

Correct answer: Hydrogen ion

The governing concept is the acid–base behavior of indicators. Blue litmus turns red in an acidic medium, showing that the solution has an excess of hydrogen ions, represented in water as hydronium ions. Therefore option A is the best answer. Hydroxide ions indicate basicity, while sodium and chloride ions alone do not establish that the solution is acidic; colourlessness is also irrelevant to the test.