"Where are they now? At home."
Translation:Де вони зараз? Вдома.
I'd say удома even if these are two people talking and there was a long pause after the question so you don't care what was before - consonant or a vowel. In other words, just as a start of the sentence I'd prefer to start with a vowel, удома. But this is just me, I am not sure about "rules"