a) Complementation is not the concept being followed here, as it is the gene product that is formed within the cell. It adds to the valor. Instead, it is the cross feeding that is occurring in the present situation. The product developed by one of the mutant strain would diffuse and help the other mutant strain to make use of it (which cannot be produced by this strain). This is the main reason why there was a higher growth at the ends of the streaks in the plates.
b) The mutant strain should have had a block that have formed earlier in the metabolic pathway leading to the depletion of the gene product. As a result of cross feeding, the gene product that are lacking in the mutant strain would be made available to the organisms leading to a luxuriant growth at the edges of the streaked organisms in the plate.
c) Luxuriant growth at the end of the streaks of trp E strains shows that the metabolic pathway is hindered by mutation much earlier than the other two strains. The strain which has trp D shows that there is a much reduced hindrance in terms of mutations as the excess growth is present at only one end of the streak (at the end which is closer to trp B). The strain that bears trp B is the strain that looks unhindered by mutation. Hence, this shows that trp E is the first gene present in the metabolic pathway followed by trp D and trp B respectively.
d) Since all the organisms were tryptophan mutants, the requisite supplementation of tryptophan is highly important as without which none of the mutant strains would grow on minimal media.