Making each question mandatory

You can specify that students must answer each question in the test or survey. When this option is enabled, students cannot navigate from the test or survey page without answering each question on each page.

To set that students must answer each question:

Double-click the graphic of the test or survey you want to change. Alternatively, you can right-click the test or survey in the Title Explorer and select Properties.
On the Behavior ribbon, enable the Student Must Answer Each Question check box in the Questions group for tests. On the Behavior & Results ribbon, enable the Student Must Answer Each Question check box in the Questions group for surveys.

Each question must be answered.