What should be done before covering a small mistake in poster colour?

Correct answer: Let the mistaken area dry

The governing concept is controlled correction with an opaque layer. Letting the mistaken area dry prevents the wet colours from spreading further and gives the correcting paint a stable surface. Then a suitable, sufficiently opaque mixture can cover the mark neatly, so option A is correct. Rubbing wet paint can damage paper, extra water spreads the error, and tearing the sheet is unnecessary for a small mistake.