Based on the setting of the question, a parental plant of the dwarf/white phenotype can only have the recessive genotype of ddww (Only b or e plausible). Thereby the cross is considered a test cross, in which the phenotype of the offspring depends solely on the alleles from the non-recessive parent, in this case, the tall and violet plant (Genotype DxWx). Within Mendelian concept (not considering recombination), a test cross yielding all four possible phenotypes at a even proportion requires an exhaustive allelic pool from the non-recessive parent. Among b) and e), only genotype DdWw from b) is capable of fulfilling this requirement, as it gives four different gametes with alleles DW, Dw, dW, and dw.