"What is under the table?"
"桌子下有什么?" should also be accepted since 下面, 下 and 下边 are interchangeable in this instance. Please see the following link for a comprehensive explanation on expressing location:
You are correct but they don't offer all the characters to write that.
I write down in Pinyin, English and Chinese characters, every question and answer they ask.
Surprisingly they ask exactly the same question earlier and the answer is just what you have written. "桌子下有什么?"
I don't know why they changed it here for any other reason than attempting trick students. It's a hard enough course as it is, why do this I wonder?
Duolingo breaks the link to the Grammar Wiki because they think "..." can't be part of the title, and they think the _ representing space in the URL is supposed to represent formatting, and, just, oy.<pre>
You can just "escape" the formatting characters by preceding them with a backslash:
Of course it may not be any easier, but it avoids the yellow highlighting.
I'm not a native speaker, but I think it's okay, for what it's worth, with a couple of caveats.
One is that "什么东西" can be used to mean something like "what the heck...", so your sentence could perhaps be interpreted to mean "What the heck is under the table?"
The other is that "面" is "face", so using it may tend to imply that whatever it is is stuck on the underside of the table, rather than simply under it in general. On the other hand, though, I don't think "下面" is necessarily used all that literally.
I'd also like to hear from a native speaker.