Final answer:
Sulla wanted Caesar to divorce Cornelia to weaken him politically by severing his ties to his enemies; this demand reflected the use of marriage for political alliances in Roman society.
Step-by-step explanation:
The reason why Sulla wanted Caesar to divorce Cornelia was c) Sulla wanted to weaken Caesar politically. The dynamics of Roman political marriages were often influenced by alliances and power struggles. In the Roman context, marriage was not only a personal but also a strategic political tool. Sulla, having established himself as the dictator of Rome, used marriage to reward and secure allegiance from his supporters and to weaken his adversaries. In Caesar's case, divorcing Cornelia, who was connected to one of Sulla's opposing factions, would have been a political move to sever Caesar's ties to his enemies and to display his loyalty to Sulla. However, it is notable that Caesar defied Sulla's order to divorce Cornelia, indicating his personal commitment to her, as well as a refusal to submit to Sulla's political will at this juncture.