Hi Ellen,
what is the difference between a passive and a causative? Sorry, I'm not familiar with the grammar term 'causative', perhaps someone else can offer an opinion.
I know that a causative describes something being done for someone by someone else.
Mary cooked dinner for Tom fits this definition. Is that what you mean?
And I think that a causative can only be found in an active sentence.
Dinner was cooked for Tom by Mary is the passive version, and seems to have the same meaning as the active version.
I think you need to look carefully at your definition of a causative, and at some examples of it. As I said, perhaps someone else here can offer you more information.
Good luck with the exam,
Clive