Final answer:
Paulette is most likely thanking the person for taking Rufus for a walk.
Step-by-step explanation:
The most likely reason why Paulette is thanking the person she is speaking to is that (B) the person has just taken Rufus for a walk. Paulette expresses her gratitude for the person's act of walking Rufus on her behalf. This implies that the person has done a favor for Paulette by taking care of her dog, which she appreciates.