If you don't know the answer you must select *any* answer, in that case you have possibility to guess and answer correctly
If you don't select any option you fail this question for sure.
Mikalai is exactly right. As an additional note, in *every* case, you must select the correct number of answers. If a question asks "choose two", any number of selections other than two is a guarantee of no mark (and there's only one mark per question, too, btw).