To solve this problem, we need to analyze the properties and formation of cross-linked or network polymers.$\newline$Cross-linked polymers are characterized by the following:$\newline •$They are formed by the linking of polymer chains through covalent bonds, creating a three-dimensional network.$\newline •$These polymers are typically formed from bi-functional or tri-functional monomers, which allow for the formation of cross-links.$\newline •$Examples of cross-linked polymers include bakelite and melamine, which are known for their rigidity and thermal stability.$\newline$Let's evaluate each option:$\newline$Option 1: Examples are bakelite and melamine.$\newline$• This statement is correct because both bakelite and melamine are well-known cross-linked polymers.$\newline$Option 2: They are formed from bi- and tri-functional monomers.$\newline$• This statement is correct as cross-linked polymers require monomers with multiple functional groups to form the network structure.$\newline$Option 3: They contain covalent bonds between various linear polymer chains.$\newline$• This statement is correct because the cross-links are covalent bonds that connect different polymer chains.$\newline$Option 4: They contain strong covalent bonds in their polymer chains.$\newline$• This statement is incorrect in the context of distinguishing cross-linked polymers.$\newline$• While it is true that cross-linked polymers have strong covalent bonds, this statement does not differentiate them from linear polymers, which also have strong covalent bonds within their chains.$\newline$Therefore, the incorrect statement is Option 4, as it does not specifically address the unique feature of cross-linking in these polymers.