"Please return it soon."
1st of all, すぐ does not mean "soon", it means "immediately". 2nd "それを" can be entirely omitted here, so not using it should not be marked as wrong. Noone in Japan would use that!
1st 'Soon' also is translated as 'すぐ'.
2nd It is good that 'それを' is translated. Because 'それは' is as the word 'it' in this sentence. 'it' is 'something to what'.
eg. 'Please return the book soon'. 'the book' should be translated. As same as 'it' in this sentence should be translated.
There are some cases what 'it' is not entirely translated.
eg. 'It is rainy. /雨です。No one would say 'it/それ' about 'weather/天気'.
I disagree. I think it's not necessary to translate the "it" because in most contexts it would be understood what is being talked about. It's more natural in Japanese to say just すぐに返してください.
Including "sore wo" in the sentence implies a context in which it is necessary to point out what is to be returned. Japanese normally does not express what can be undestood in context. If it is clear what is to be returned "sore" is most likely to be omitted.
Why is それ "it" in this case, while it's "that" in every other example?