In planet Aggieus DNA replication is known to be conservative. Researchers kept replicating alien bacteria in media supplied with 15N and then isolated a single bacteria and placed it into 14N containing media.

After three divisions how many bacterial cells would contain 15N DNA?

1 bacterial cell would contain 15N DNA

Explanation:

According to conservative model of DNA replication, the two DNA strands do not separate from each other and direct the synthesis of entirely new DNA molecule.

Here, a single bacteria had 15N DNA strands and then it was placed in N14 media. All the subsequent divisions would utilize 14N.

After one division: One 15N DNA and one 14N DNA

After two divisions : One 15N DNA and three 14N DNA

After three divisions: One 15N DNA and seven 14N DNA

Hence only one bacterial cell will contain 15N DNA