Final answer:
A travel agent does not need to communicate with the flight attendant when planning out a trip for a client; they need to interact with the client, the hotel registration, and the rental car agency.
Step-by-step explanation:
When planning out a trip for a client, a travel agent communicates with multiple parties to ensure a smooth experience. However, the travel agent does not need to communicate with option C, the flight attendant. Communications regarding flights are usually handled by airline customer service or a ticketing agent before the trip; the flight attendant's role is primarily during the flight itself to ensure passengers' safety and comfort. Therefore, the other options are entities that the travel agent would need to coordinate with: The client themselves (A) to understand their travel preferences and needs, the hotel registration (B) to book accommodations, and the rental-car agency (D) to arrange transportation.