The new question close reasons have shuffled things a bit.
The best thing is to vote to close the question if you have earned that privilege so that it hits the close queue for others to review. Since not all users have close votes to cast, the flag system is there as well:
It would be nice if the other option showed up for "It doesn't belong here, or it is a duplicate…" so you might need to explore the second option and realize the system doesn't let you use other for a flag except for the first screen you see.
It would be best to choose other in the screen above and explain what you find wrong. If you felt strongly that the question would go to chemistry, that would be great, but the main thing would be to express that it's not a good fit here and let the moderators sort out what to do with your general flag of the above question.