The answer to your question would be that the best sentence variety using transition words is the following one: Ande was the only one who had praised his ability; therefore, Arthur gave her his two best drawings.
The adverb "therefore" means as a result or consequence, or "for that reason". Moreover, it is also used to introduce a logical conclusion from that fact, reason, or as a result. In this case, Anda prasing Arthur's ability is the reason why he gives her his two best drawings.