Radio active cytosine was added to an actively growing culture or E. coli bacteria. Which of the following would be the result if a cell replicated once in the presence of this radioactive base?

If replication happens once, then it will create two daughter cells. Both cells will contain a DNA which is radioactive. This happens because prokaryotes like E. coli exhibits semiconservative replication, wherein each daughter cell receives one template strand from the original molecule, in this case, it's made up of the radioactive cytosine.