Can questions be made compulsory?

Yes.  To make a question compulsory:

1. Create a new question, or edit an existing question.
2. Click on Advanced options.
3. In the Optionality & dependency section, choose Required.
4. Click Add question (if adding a new question) or Save (if editing an existing question).

A red star will appear next to the question text to indicate that the question is now required (compulsory).  Respondents will be unable to move from one survey page to the next (or submit their answers at the end of the survey) until they have provided a response to all required questions.

You do not have to have any compulsory questions in your survey if you do not want to.

Note: Grid questions contain several questions within a table.  The optional/required settings must be set for each question within a grid individually.