<p>Mediators were standing by, prepared (to intervene in) the labor dispute (even though) both sides (had refused) earlier offers (for) assistance.</p>
<p>Answer is "for". WHY? And what would replace for?</p>
<p>Instead of “for,” it should be “of.”</p>
<p>[I assume it was “mediators”}</p>
<p>Oh yes! Of. Didn’t think of that. Actually English isn’t my first language, so identifying such errors is kinda difficult for me. Thank you very much for the help! (And yes it’s mediators)</p>
<p>This is where I repeat my little sermon: the best way to improve your ability to answer this kind of question is to read as much good written English as possible. I can’t tell you exactly why “offers” takes “of” in this situation–I just know that it does.</p>