Quite a bit of general knowledge is involved in classification type questions.
Questions that contain pictures and objects often only involve observation.
Check the options for more than one correct answer, some answers are more valid than others.
If there is another correct answer avoid it if it is too broad or narrow in scope.
Broad scope example: Avoid creating a class out of green things in a situation where there are only two classes examined, fruits and vegetables.
Narrow scope example: Continuing the above example, avoid creating a scientific names class, in case there exists such a similarity/difference among the items.