A second poster-colour layer is lifting the first layer. What may be the main cause?

Correct answer: The first layer was not fully dry or the brush was too wet

The governing concept is controlled layering in poster-colour painting. If the lower layer is still damp, or if the brush carries excess water, the moisture reactivates and loosens the paint underneath. Repeated brushing then lifts that layer instead of covering it. Therefore option A is correct. A clean brush, a flat palette, or white paper does not normally cause lifting; the practical remedy is to let the first coat dry and use a moderately loaded brush.