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152 five letter words begin with UN, and almost every one of them is the prefix attached to a three letter root.
D leads the third box at about nine percent, then B and S at eight and I and A at seven. Nothing dominates.
That flatness is the prefix at work. UN attaches to whatever root follows, so the third letter carries no pattern of its own.
UNDER, UNTIL, UNION, UNITS and UNITY are the most common entries, and the first two are among the most used words in English.
UNION, UNITS, UNITY, UNITE and UNIFY all run UNI, which is a genuine family rather than a prefix plus a root.
Confirming that I is the single most useful narrowing available on this page.
UNITE is the guess to open with. It covers the one real family here and its T and E are common enough to help even when it misses.
UNDER is the more frequent word, but it tells you far less about what else might fit.
Because it identifies a prefix, not a word shape. The three boxes after it can hold almost any workable root.
It is a real word and its Z is unique on this page, so if a Z turns up anywhere the answer is almost certainly UNZIP.
Only the last box. Both sit in the UNI family, so a guess on either narrows the other to one option.