I have three questions.
1) in similar exercizes, "during" is also given as a valid translation, so why is, "Who was here during the night?" Not accepted.
2) "Who was here in the night?" doesn't really sound natural anyways. More natural questions would sort of specify which night. E.g., "Who was here last night?", "Who was here that night?", "Who was here overnight?" 3) Why is the recommended correct English translation no longer appearing with the Ukrainian exercize text at the top of the page? Thank you
There is an у—в alteration, affecting both the prefix у-/в-, and у/в as a preposition (as well as the і—й alteration). Accoding to the orthographic rules, this sentence should have «уночі» because «тут» ends in a consonant.
However, this rule about alterations is not often followed, because AFAIK it's not taught in schools, the rules are different between dialects and because of the Russian influence. So you're only likely to see that alteration in professionally proofread texts.
Duolingo seems to ignore this alteration altogether, and uses the vowel (у- and і-) everywhere.
"Who was here at night?" is not British English. I would say "Who was here over night?" or "Who was here during the night?", or, as stated below, "Who was here in the night?". "at night" is used for a continuous or repeated activity. For example, "I sleep at night", or "I like to go for a walk at night", or "Bats fly at night".