If a quiz is found to have a question with an incorrect answer or you decide later to exclude a question due to confusing wording or any other reason, you can correct and re-grade the quiz.
There are two options for excluding/dropping a question from a quiz: 1) give everyone credit for that question or 2) change the question point value to zero. The former might not possible for question types other than multiple choice. The latter option slightly increases the point value of all the other quiz questions, unless the Max Grade is also adjusted.
To give everyone credit for a multiple choice question:
Or to completely exclude the question: