Translation:The boy's attempt was dangerous.
I looked it up in the dictionary (before typing my comment above) and it seems that try is mostly used as a noun in plural, e.g. It took five tries before he succeeded. Or maybe "you still have one try left"? But it sounds awkward in the sentence above, to me and a few other people at least. :) You can always report it to alert the course contributors to it, of course.