Clearly, there is a variation in these marine snails so 'No variation' is definitely not an answer. Artificial variation is when humans interfere with their genetic engineering that modifies the DNA of the organism. Assuming that this is natural, then this is not the answer.
Environmental and genetic variation can be easily confused with. Genetic variation is more on the genotype, the composition of the genes that form the physical traits. While environmental variation is more on the phenotype, or the trait that you see and observe. However, there can be genes that are inherited, but due to the influence in environment, they don't show up. So, assuming that these colors are consistent from birth, then the answer would be genetic variation.
