The lower layer lifts while correcting a dry poster colour area. What may be the cause?

Correct answer: Repeatedly rubbing the same spot with a wet brush

The governing concept is reactivation of a water-based layer. Poster colour may soften again when exposed to water, especially if a wet brush repeatedly scrubs the same place. This action lifts and mixes the dry lower film, so option A is the likely cause. A thick correction layer can cover the surface, mounting provides support, and a clean brush is generally helpful; none normally causes the lower layer to lift by itself. Use fewer strokes and allow drying between corrections.