—Surah al-Kahf, 18:84
They ask you about the story of Dhul-Qarnain.
The response given is to recite to them the story, starting with:
—Surah al-Kahf, 18:85
[We established him in the earth.]
and it was completed to the extent needed. This Dhul-Qarnain is the one mentioned in Daniel 8:4.