Which of the Following Defines the Essential Gene Set of an Organism

Which of the following defines the essential gene set of an organism such as a species of yeast?

A) genes whose mutations result in haploinsufficiency
B) the fewest genes that an organism needs to divide
C) genes that, when mutated, are conditionally lethal in special environments
D) genes whose deletion alleles are lethal
E) genes that, when mutated, reduce the growth of a heterozygous organism