Report it from the question itself. There is a report control on the question, and it sends the exact item, your answer and the recorded correct answer along with your note, so nobody has to reconstruct which question you meant.
This is the highest-value thing you can send us. Several real content defects have come in this way, including questions that accepted only one of two valid answers.
For anything else broken, the Help button in the corner takes a screenshot attachment.
Did this answer it? If not, the Help button in the bottom-right corner of any page reaches a person, signed in or not.