I agree that, altogether , Sophie’s experiences last winter were beneficial to her development.
Explanation:
Although very similar, the terms "Altogether" and "All together" mean very different things.
"All together" means to put everything together in one place, that is, everything is together, united.
The word "Altogether" has an adjective function and can have three different meanings. It can mean "totally", "in total" and "in general". This shows why this word is the most appropriate to fill in the blank in the question. Because the meaning of "Altogether" maintains the coherence and cohesion of the sentence enabling the understanding of the reader.
