"She doubts the reality of things."
Translation:Elle doute de la réalité des choses.
Some verbs require a "de" when they are followed by a noun, here is a list of some of them: http://french.about.com/od/grammar/a/preposition_de_3.htm
There isn't really any rhyme or reason to it.
I got this phrase with multiple choice of answers. In the English, there is no "the" before "things", so I chose "de choses". The right answer is somehow "des choses" (=de les, right?).
I am so tired of this "the" issue. Guys, what is it really that you want your learners to learn?)))))