親になっ___、親のありがたさが分かった。
Q. Which completes 'only after becoming a parent did I understand'?
Translation
Only after becoming a parent did I understand what my parents did for me.
Explanation
〜てはじめて means a realization that occurred only upon experiencing something: なってはじめて分かった. たとたん is a sudden immediate event, and てからでないと needs a negative consequence clause.