I would say that the best answer is B, it helps create a gentle and sentimental tone.
The girl is remembering about the hugs she received from her grandmother, which made me think of nostalgia. A synonym for sentimental is nostalgia.
It definitely does not create a sorrowful and sad tone because nowhere in the passage does it state the character feeling some kind of sadness.
It definitely does not create a regretful tone because the answer choice itself doesn't make sense. The passage is talking about her receiving hugs, so her feeling regret wouldn't make sense.
