Genome-wide association studies (GWAS) are primarily employed to identify genetic variations associated with complex diseases caused by multiple genes, making option c) diseases caused by multiple genes the correct answer.
GWAS facilitate the exploration of common genetic variations across individuals with certain traits or diseases compared to those without. These studies have been particularly instrumental in uncovering the genetic basis of complex conditions like diabetes, heart disease, and various forms of cancer, which typically involve interactions among multiple genes.
While single-gene inherited diseases (option a) can also be studied, these are not the primary focus of GWAS, which are more geared towards polygenic conditions. Viral diseases (option b) and diseases caused by environmental factors (option d) may interact with genetic predispositions but do not fall within the conventional scope of GWAS, which seeks to link genetic markers with multifactorial traits.