The correct answer is "Hfr strains have the F plasmid integrated into the chromosome".
This is in the context of horizontal gene transfer in bacteria. This gene transfer consist of transferring an "F plasmid" in a process called bacterial conjugation. Bacteria with the F plasmid are termed as "F+ strains". The key difference between an Hfr strain and a F+ strain is that while they both have the F plasmid; the Hfr strain have the F plasmid integrated to its chromosome while the F+ strain have the F plasmid not integrated to the genetic material and only freely floats in the cytoplasm.
