"What makes them efficient?"
Translation:Vad gör dem effektiva?
Does "effektiv" also mean effective? I ask, because in English, effective means something does what is should or works properly, whereas efficient means something works quickly and easily. Something that is efficient is always effective but something effective is not always efficient.
It's the object form of the pronoun de here, the subject being what. To have de would be like saying What makes they perfect in English – except of course in Swedish those two words are pronounced the same way, so many native speakers mix them up.
Alternatively, if de were to be an article, you'd get a grammatically correct sentence meaning What are the efficient doing? (this would never be anyone's first interpretation of this sentence though)