Among the following, the correct statement(s) about polymers is(are)

The question asks which of the given statements about common polymers are correct, so we shall check each statement against standard facts from polymer chemistry.

Case A: Polymerisation of chloroprene (2-chloro-1,3-butadiene).
Natural rubber is $$\left( -CH_2-C\left(CH_3\right)=CH-CH_2- \right)_n$$, i.e. a cis-1,4-polymer of isoprene (2-methyl-1,3-butadiene).
Chloroprene, $$CH_2=C(Cl)-CH=CH_2$$, on 1,4-addition polymerises to neoprene (polychloroprene), an important synthetic rubber.
Hence polymerising chloroprene does not give natural rubber. Statement A is incorrect.

Case B: Preparation of Teflon (polytetrafluoroethene, PTFE).
Teflon is obtained by free-radical polymerisation of tetrafluoroethene, $$F_2C=CF_2$$, using ammonium (or potassium) persulphate initiator at around 350-400 K and high pressures (≈ 100-150 atm).
Therefore the statement exactly matches the industrial process. Statement B is correct.

Case C: Nature of PVC (polyvinyl chloride).
PVC consists of $$\left( -CH_2-CH(Cl)- \right)_n$$ chains with no extensive cross-linking; when heated it softens and can be remoulded. Such polymers are called thermoplastics.
Hence the statement is correct. Statement C is correct.

Case D: Polymerisation conditions for ethene yielding HDPE.
At 350-570 K and 1000-2000 atm in the presence of peroxide initiators, ethene forms branched chains leading to low-density polyethylene (LDPE).
High-density polyethylene (HDPE) is produced at 333-343 K and 6-7 atm using Ziegler-Natta (TiCl4/Al(C2H5)3) catalyst, not under the conditions mentioned.
Thus the given conditions do not give HDPE. Statement D is incorrect.

Only statements B and C are correct.

Option B (Teflon is prepared from tetrafluoroethene by heating it with persulphate catalyst at high pressures), Option C (PVC are thermoplastic polymers).