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 that, when mutated, reduce the growth of a heterozygous organism
B) genes whose deletion alleles are not found among haploid offspring
C) the fewest genes that, when mutated, result in slow growth in optimal environments
D) genes whose mutations result in haploinsufficiency
E) genes that, when mutated, are either lethal or conditionally lethal in special environments