Regarding cross-linked or network polymers, which of the following statements is incorrect ?

Answer: 4. They contain strong covalent bonds in their polymer chains.

Cross-linked (network) polymers are formed from bi- and tri-functional monomers; examples are bakelite and melamine-formaldehyde. Their linear chains are joined by covalent cross-links.

Statements 1, 2 and 3 describe this correctly. Statement 4 says they contain strong covalent bonds in their polymer chains; the published solutions treat this as the incorrect statement, since the defining feature is the covalent bonds between chains rather than within them.

So the answer is option 4. Note: the wording is loose and statement 4 is arguably true, so this question is ambiguous; we follow the published solutions.