E. coli cells grown on 15N medium are transferred to 14N medium and allowed to grow for two more generations (two rounds of DNA replication). DNA extracted from these cells is centrifuged. What density distribution of DNA would you expect in this experiment?

One low density 14N/14N band up, and one intermediate  heavier 15N (template)/14N( for synthesis)  of higher density band  below.

This is because in the first generation the light 14N and  heavy 15N  were distributed in equal proportion  in the bacterial so that each  strand of the double DNA  consists of half heavy 15N template and half light  14N. Therefore all the strands are intermediate in composition.

According,to semi conservative replication,  after second generation  one half of the lighter N14 strands  will serve as  the template, while the other half  N14  strands  will be used for s DNA synthesis. While   one half of  15N strand will serve as  the template , and one lighter 14N will serve new synthesis of DNA to formed intermediate band,

the correct option is option d