Final answer:
Dual agency in real estate is when one agent or broker represents both the buyer and seller in the same transaction, encompassing both scenarios 'a' and 'b' listed in the question.
Step-by-step explanation:
Dual agency occurs in real estate when one agent or broker represents both the buyer and the seller in the same transaction. Therefore, the correct answer to the question 'Dual agency exists when:' is both 'a. two agents from the same office represent the buyer and seller in one transaction' and 'b. the broker represents more than one party.' This is because a scenario where two agents from the same broker represent different parties can also constitute a dual agency if the broker has not adequately separated the agents' representation of each client. It's essential for both parties to be fully informed and agree to this arrangement due to the potential for conflicts of interest and to ensure informed decision making in the market.