Answer:
Natural selection is selecting for extreme phenotypes. Over time their frequency will increase in population and frequency of intermediate phenotype will decrease in population.
Explanation:
This is a classic case of disruptive selection where both the extreme phenotypes are favoured over the intermediate phenotype. Since black snails can hide on black rocks and white snails can hide on white rocks, snails having these colours have better chances of survival.
Grey snails wont be able to hide on either type of rock so they will be targeted more by the predator and their population will decrease. Eventually, the extreme black and white coloured snails will increase in number and intermediate grey coloured snails will decrease.