This question is concerned with organisms that can or cannot live without exogenous (added to the environment) chemical X. By definition auxotrophs are those that need some essential nutrient to be added, prototrophs can synthesize their own chemical X. Also, we need to know what is
"complementation group". By definition, that is pair of mutants that have same phenotype (say, they are auxotrophs for nutrient X), but when crossed will produce wild-type (prototroph) progeny. That means that mutations in these mutants occurred in different genes, and each parent "complemented" each other, supplied wild-type gene to correct for another parent's mutation. "Complementation group" is a group of genes that do not compliment each other. By definition, each gene can be in its own complementation group.
From data, mutants 2 and 4 do not complement each other, so are mutants 3 and 5. All what's left is mutant 1, which by definition doesn't complement itself, hence in its own group.
Answer is C
This is also a useful link: http://courses.washington.edu/gensc371/lecture/jan22.pdf
