What is the most suitable use of opaque acrylic paint?

Correct answer: Covering the lower layer to create solid forms

The governing concept is opacity: an opaque acrylic layer reflects enough light and contains enough pigment to hide much of the colour or drawing beneath it. It is therefore useful for building solid shapes, correcting areas and placing clear highlights over lower layers, making option A correct. Transparent glazes require a thin, diluted or transparent mixture, while soaking paper and cleaning brushes are preparation or maintenance actions, not uses of opaque paint.