Final answer:
Buy side systems are a combination of supply side systems and sell side systems, used by businesses to manage procurement processes. Option 3.
Step-by-step explanation:
The category of e-business system called buy side systems falls under the category of both A and B (3). These systems are used by businesses to manage their procurement processes, including sourcing, purchasing, and contract management. They involve interactions between the buyer and the supplier, making them a combination of supply side systems and sell side systems.