Idiom/collocation – identify the noun that fits the set phrase "on the ______ of adulthood". Sentence: "Many of the advances of civilisation have been conceived by young people just on the ______ of adulthood." Choose the word that correctly completes the idiom.

Given data

• We need the standard noun used with "on the ____ of" to mean "at the start of".

Concept/Approach
The idiom is on the threshold of, meaning about to begin a new stage.

Option analysis
threshold — correct idiomatic choice: "on the threshold of adulthood." ✅boundary — a dividing line, but the set phrase is not "on the boundary of" for stages of life.peak — means highest point, not beginning.horizon — metaphor for the limit of vision/knowledge; not the standard idiom here.

Usage note
Common collocations: threshold of adulthood/success/a new era.

Final Answer
threshold