Plasmid is the correct answer, as plasmids carry antibiotic resistance and are involved in conjugation, a form of sex in bacteria whereby the bacteria produce a sex pilus (sort of like a penis) and use it to exchange DNA with neighboring bacteria. It takes around an hour for bacteria to have sex, in order to exchange almost the whole genome. In that sense, humans have a lot to learn from bacteria! this also explains why antibiotic resistance spreads so quickly, because the plasmid DNA is very mobile and is often present at hundreds of copies per bacterial cell. Farm animals are often fed a diet heavily laced with antibiotics, which causes antibiotic resistance to spread rapidly. However, plasmid DNA is very useful for DNA vaccines and gene therapeutics, two major classes of emerging biopharmaceuticals.