"Where were you so long?"
Translation:Wo seid ihr so lange gewesen?
They have accepted my answer "Wo bist du so lang gewesen?" Is it ok without the "e" in "lang"?
No. In a word for word translation, wo translates as where and vice versa. On the other hand, "Where were you so long?" is different to "what took you so long?". So, their translations would be different.
In this case, "long" in "so long" is an adverb, so "lang" and "lange" are adverbs. "Lange" is the usual word while "lang" is a regional one. Note that "lange" is also a declination for the adjective "lang".
The distinction between the past tense (Präteritum) and the present perfect (Perfekt) is not as strict in German as it is in English. Mostly, it's just a matter of style. That's why an English sentence in the past tense ("Where were you so long?") can either be translated as "Wo warst du so lange?" (Präteritum, i.e. German past tense) or "Wo bist du so lange gewesen?" (Perfekt, i.e. German present perfect). Both translations are correct.
"Wo sind sie so lange" accepted also. It seems all versions are accepted. :)
It's still weird to me that it accepts lower case "sie" when you're using it as formal "you".
Why is sein used and not haben? I thought sein was used if there were movement from A to B