How would you prepare stable polymers from formaldehyde?

It is produced principally by the vapour-phase oxidation of methanol and is commonly sold as formalin, a 37 percent aqueous solution. Formalin may be dehydrated to trioxane, a crystalline trimer, or to an amorphous polymer, paraformaldehyde, which is a convenient source of gaseous formaldehyde.

the answer is c. nacl

it’s previous models   because when i picked that son of a gun’s answer it was wrong look for previous models because they might make it a different letter