White clover (Trifolium repens) can be either cyanogenic, that is, it produces hydrogen cyanide; or non-cyanogenic, that is, it does not produce hydrogen cyanide - VCE - SSCE Biology - Question 21 - 2005 - Paper 1

The correct answer is A. grazing action of snails and slugs. This is because snails and slugs consume cyanogenic plants, which decreases the prevalence of non-cyanogenic clover plants in the population. In environments where snails and slugs are prevalent, the selective pressure would favor the survival of cyanogenic plants, leading to a lower number of non-cyanogenic plants.