Step 1: Identification
The structure shown in the image (Question 19) is Teflon, represented by the repeating unit $-(CF_2-CF_2)-_n$.
Step 2: Monomer
The monomer for Teflon (Polytetrafluoroethylene) is Tetrafluoroethylene ($CF_2=CF_2$).
Step 3: Conclusion
The monomer is Tetrafluoroethylene.
Final Answer: (B)