I don't think it's 1 unit chosen randomly. It is usually based on the leadership of the lost stack. For example, in my Pala hard game against baal I lost 400 Priests, about 250 Inquisitors, 70 Archmagi and around 150 horsemen and some Paladins. The hero resurrected the 150 horsemen in the end, which I consider to be the most appropriate choice.
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