도화선이 · 도화선을
fuse; trigger, spark (that sets off an event)
Example
그 사건이 시위의 도화선이 되었어요.
That incident became the trigger for the protests.
Word parts
도 lead+화 fire+선 line
Literally 'a line that leads fire', the fuse of a bomb.
Good to know
Most often used as X의 도화선이 되다, 'to become the trigger of X', in news reports.