If we assume that DNA replication is according to the conservative model. Starting with 14N14N (lighter) DNA, and after two generations in the 15N medium, bacterial cells will produce the following band/s in density-gradient centrifugation.  

Meselson and Stahl transferred the bacteria from 15N medium to 14N medium. They allowed them to replicate for two generations. DNA extracted from these cells is centrifuged. The DNA obtained after the first round of replication is