A short fragment of the polymer chain (two repeat units, end groups shown) is drawn. Which of the structures shown is the monomer that gives this polymer?

A backbone in which every carbon carries two fluorine atoms is polytetrafluoroethene (Teflon). Its monomer is tetrafluoroethene, $\mathrm{CF}{}_2=\mathrm{CF}{}_2$; the chain end F atoms come from the initiator or chain end. Hexafluoropropene would leave a $\mathrm{CF}{}_3$ branch on the chain, and vinylidene fluoride or chlorotrifluoroethene would not give a fully fluorinated, Cl-free repeat unit.