Answer: O Plants can add beauty and warmth to a home, they also provide a good source of oxygen.
Explanation: The sentence "Plants can add beauty and warmth to a home, they also provide a good source of oxygen" is grammatically correct and effectively combines the two clauses into one sentence. The conjunction "they also" serves to connect the two ideas and creates a clear, concise statement. The other options use either "in addition" or "and," which don't provide as smooth a transition between the two clauses. The semi-colons in the other options also break up the flow of the sentence, making it less readable and less smooth.