For many of these questions, the idea is that the word enough is not used in the same way as too or very are used; you simply don't say enough difficult, or enough expensive. When modifying an adjective, the word is enough is used after the word it modifies, not before:
He's old enough to stay out past midnight.
He's skilled enough to ski down the expert trail.
If you want to use a word before the adjective, you can use sufficiently:
He's sufficiently skilled to ski down the expert trail.
or, you can restructure the sentence, and use enough to modify a noun:
He has enough skill to ski down the expert trail.
As for this example problem:
______ so we didn't get it.
a. expensive enough
b. too expensive - correct
c. enough expensive
As I mentioned previously, I can't think of an instance when I would ever say "enough expensive." As for "expensive enough", there are times I might use that wording, but not within the "It was
___, so..." construct. However, I might say:
We didn't go out to dinner after the show; the tickets to the play were expensive enough.
As for this one:
_____ to read; I don't understand it at all.
a. enough difficult
b. too difficult - correct
c. difficult enough
Again, (a) is just downright awkward. The third option almost works, but too difficult matches better with the speaker not understanding the reading material. If, however, the sentence had said,
_____ to read; I think it'll take me forever to finish that book!
then (c) might be more acceptable, because the word enough can be used for emphasis, where "difficult enough" can mean, essentially, "very difficult."
As for this one:
_____ to sort things out.
a. late enough
b. too late - correct
There's nothing wrong with saying late enough, but not in this context (because it's hard to think of a context where it's too early to sort things out). Instead, one might say:
It's not late enough in the day yet to start selling our bread for half price.
(Admittedly, I'd probably opt for "too early," instead of "not late enough," but, nevertheless, that sentence is grammatical.)
I hope these explanations help you out. It's hard to tackle all six of these at once – which is why some may have voted to close the question initially. Hopefully, though, I've explained enough, so that you can figure out the ones I didn't address.
One last thought, about your comment (that this must be "the worst site for English help"): to that, I say, "Enough, already." I think there are enough helpful people on EL&U that such comments are unwarranted and unnecessary.