I've come across this question in a mock exam online. It's apparently a true/false kind of question.
Development team is self-organizing, therefore their composition can be changed mid-sprint.
My answer was TRUE but according to the website the correct answer is FALSE.
Here is the explaination from the website:
Development team is self-organizing, but it doesn't mean that they can change their composition mid-sprint. The composition change can happen between two sprints.