Answer:
The desk was covered with books, the rug was littered with games, and the bed was covered with clothes; however, Sam claimed that his room was not messy.
Step-by-step explanation:
Transitional words are used to link ideas in a sentence, paragraph, or a text. As the name suggests, they help the speaker/author transition from one idea to another smoothly, even if the ideas are different or opposite.
In the sentence we are studying here, the writer describes a room that is clearly messy to most people's standards. Books, games, and books are carelessly tossed all around. It would be only natural for Sam to agree that the room is far from being neat. However, he does not. We need "however" to make this transition because it connects opposing ideas. The other options, "therefore", "while", and "finally" would not convey this discrepancy between Sam's opinion and reality.