Final answer:
The thioester bond is an important functional group located in the C3b subunit of C3, having a role in the immune system's complement pathway.
Step-by-step explanation:
The thioester bond is of significant importance in biochemistry and is located within the C3b subunit of C3. The component C3 of the complement system can be split into two fragments: C3a and C3b. The thioester bond is found in C3b, where it plays a key role in the complement system's ability to fight infections by facilitating the attachment of C3b to the pathogen's surface, which in turn aids in the process of opsonization for phagocytosis. Therefore, the correct answer is b) C3b.