Haploids are preferred over diploids for mutation studies because

Dominant mutations are the one that expresses themselves in heterozygotes as well as in homozygous dominant genotypes; thus a cross between two pure breeding genotypes produces the dominant phenotype in F${}_1$ hybrid. This means that dominant mutations are expressed in successive generations of diploids; options C and D are incorrect in respect to the question. Recessive mutations are suppressed by the presence of wild-type dominant alleles in diploids and are expressed only in recessive homozygotes; their detections need development of hybrid line by the cross between two pure varieties followed by selfing of hybrid to obtain recessive homozygotes in F${}_2$ generation. Since haploids carry a single copy of allele for a gene, they express both dominant and recessive mutations in successive generations which makes option B incorrect and option A correct. The correct answer is A.