That is not true. You always have the option to choose whichever answer for a multiple choice question you want to be the "correct" answer. I am assuming that you are using current Classic Quizzes. If this is the case, check out the screenshots provided in this Guide:
Scroll about half-way down the page, and look for the heading "Change Correct Answer". This will show you how to change the green arrow to the left of possible answer choices to a different location ... marking that item as "correct". Just make sure that you click the "Update Question" button when you are done, and then scroll to the very bottom of your quiz when you are completely done to make sure you save all of your changes.
If you are using New Quizzes, you will see that there is a round circular icon to click to the left of any potential answer choice to mark it as the "correct" answer:
This Guide also shows you how you can shuffle answers in a multiple choice question...if this is something you want to do.
(Please note that you might not have access to New Quizzes at your school. Not all schools have enabled New Quizzes just yet. If this is the case, you would need to reach out to your school's Canvas administrator to have a discussion with him/her and see what the plan is to making New Quizzes available to you and your fellow instructors at your school.)
I hope this will help a bit. Let Community members know if you have any other questions about this...thanks!