Assignments are given in college to help you learn and/or to cement the knowledge that is imparted during the course.
The first place to start is your course notes - they should supply all of the knowledge required to answer this assignment. If they don't then you should talk to your tutor.
Talking to your tutor is a good idea anyway - that is what they are there for! They should be able to guide you in the right direction.
Sometimes, however, an assignment is given to encourage you to learn how to do research. Posting a question on this forum does not constitute "research".
Google (or Bing etc) is often a good place to start... post into your favourite search engine "the different classes of schemas that are popularly used for modelling data warehouses" and it will give you around 140k results in less than a second. Of those that I found
this[
^] gives a good explanation and includes references for further reading.
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