I'm a bit confused as to this "Qu'est-ce qui" form. Why isn't it "Qu'est-ce que" here? I'd think that would translate (literally) to "What is it that makes them efficient?" The correct answer here reads to me (again literally) as "What is it who makes them efficient?"
"qu'est-ce qui" is used when "qui" is subject: "qu'est ce qui fait ce bruit ?" (what makes this noise?).
"qu'est-ce que" is used when "que" is object: "qu'est-ce que tu veux ?" (what do you want?)
...and I actually understand that difference. But by using "qui" instead of "que" in this sentence, aren't you saying "who", not "what" makes them effective?
The first "qu' " makes it a thing and not a human being: "Qui est-ce qui les rend..." would be about a person.
Relative pronoun "qui" is used for things and humans when it is the subject of the [relative clause]:
qu'est-ce // qui est-ce [qui les rend efficaces] - subject "qui", verb "rend"
qu'est ce [que tu fais] - subject "tu", verb "fais", object "que"
Because you never contract "qui". I don't know if there's really a better answer than that.
What Smearedink said is indeed true and there is a reason for it: not to confuse with "que" which can be elided.
I'm just curious, but in this sentence, could "fait" be equivalent to "rend"?
Yes, that's the idea. But we don't say "qu'est-ce qui les fait efficaces ?".
That is correct, except that "interesting" is "intéressants" or "intéressantes"
"What makes them efficient" works better than the answer that was given to me when I got the answer wrong the first time. It was also accepted as an answer.
I read Sitesurf's explanation and it clicked.
"Qu'est-ce qui les rend efficaces ?" Can anyone say a sentence where this expression useful is? - irreplaceable!
Imagine I turn up at your door wanting to sell you the most efficient pen in the world for $10 a pop.