To solve this problem, we need to identify which polymer is thermosetting.$\newline$Thermosetting polymers are materials that irreversibly cure, forming a rigid structure.$\newline$Let's analyze each option:$\newline$Option 1: The structure represents polyvinyl chloride (PVC), a thermoplastic polymer.$\newline$Option 2: The structure represents polyvinyl chloride (PVC), a thermoplastic polymer.$\newline$Option 3: The structure represents nylon, a type of polyamide, which is a thermoplastic polymer.$\newline$Option 4: The structure represents Bakelite, a phenol-formaldehyde resin, which is a thermosetting polymer.$\newline$Bakelite is known for its heat-resistant and electrical insulating properties.$\newline$Therefore, the correct option is Option 4.