Final answer:
The essential features of relationship patterns in this context are that each event is linked to at least two participating agents and each event is linked to at least one other event. Therefore, the correct answer is D. A and C.
Step-by-step explanation:
The essential features of relationship patterns in this context are that each event is linked to at least two participating agents and each event is linked to at least one other event. This means that for each event, there are multiple agents involved and there is a connection between events. Therefore, the correct answer is D. A and C.