...
For assignments, click on the name of the offending question, and scroll to the bottom of the page: item parts.
(If needed) Click the green + on right hand side to expand this section.
You can choose to withdraw the question and give full credit to maintain the same total assignment grade, or to simply take it off as if it never existed.
click on the withdraw/change points tab, and select the most appropriate button for your purposes.
Withdraw no credit will eliminate the points of this question from the total points of this assignment–it is as if this question (and it's points) are erased. Withdraw full credit offers all students the points of this assignment–everyone gets it correct and the question's points are retained in the total points of the assignment.
Active means students will be expected to answer the question for the amount of points in the box (in this case, zero).
(Either action will preserve the students' submissions on the "View Submissions" page.)
Extra Credit means if this problem is correct students will get +points to their assignment, even though these points will not be counted in the denominator total sum of the assignment (hence, extra credit)
Click 'Save' to update the assignment with this item withdrawn.
You can also use this page to offer extra credit! Marking a question active/extra credit means that the points listed will not be summed in the assignment's total points, but will be granted if students get it correct. If you want students to be able to miss one question and still get a perfect score you can mark any one question extra credit, and because the assignment points will total, this will functionally reach the goal.
Another possibility on this page is to change the number of points a question is worth. Just change the points and click save.
Additionally, on the homework page you can provide feedback about the problem with the question by hitting the little yellow flag icon on the right. That will take you to a page where you can indicate how we can fix the question for future usage: Reporting an error in a question.
...