9. A real estate broker listed a home for a seller for $100,000. Later that same day, a buyer come into the broker's off...

Broker is a person who is dealing with the seller and buyers of the property to consider a transaction. He will work as an intermediary for both the parties to check on the properties for buyer or check on buyers for the sellers to sell them. 

Customer under real estate would be considered as a person who does not represented himself through agent. He is looking for some property and taking general information only in the current case.

As far as client is concern, the seller must have signed the agreement with the broker that he could represent him in front of the buyer. Broker could provide all the information given by seller to the customers when taking information.

Here the seller is considered as client and buyer is considered as customer who is just looking for some information and has no agreement with the broker to represent him.

There is also confusion as dual agency exists or not. The dual agency lies with the broker handling seller and buyer as a client on the same transactions when it is agreed that buyer is interested in buying the seller's property. But here, no such detail is being provided.

So, Option C is correct!