Teachers have the ability to define which questions on a formative must be answered for students to be able to submit their work, by setting required questions.
To set a required question:
Click the blue + sign and choose the question type
Proceed to create the question
Toggle on the "Required" button below the question

Once a question has been set as a required question, students will not be able to submit their formative without providing a response.
From the student's view, there will be 3 indicators that required questions are included in the formative:
The question will have a red label "Required"
A warning will appear over the "Submit" button stating "Please answer all required questions."
The "Submit" button will be grayed out and non-functional until all required questions have been answered

There is no limit to the amount of required questions teachers can add to a formative.