"She drinks water, because she is thirsty."
Translation:Ze drinkt water, want ze heeft dorst.
Both ze and zij are accepted in all possible answers and all combinations in this sentence, you probably made another mistake as well.
Can I put the word "heeft" at the end like it is done in german? "Ze drinkt water want ze durst heeft" sounds right to me, too.
If you use omdat to link the two parts of the sentence the heeft would be at the end. Ze drinkt water omdat ze dorst heeft. The difference being that want is a "coordinating conjunction" like en and maar, while omdat is a "subordinating conjunction". The following explains the use of want/omdat some more: www.dutchgrammar.com/forum/viewtopic.php?t=1578