<p>A (For) any mayor of a vast metropolitan area, an B (understanding of) current issues in all districts C (is) not only D (desirous) but also vital.</p>
<p>I understand that D is incorrect; it should be "desirable". </p>
<p>Regarding Choice A, I know "for" is correct because I guess here it's used for the benefit of, one of the two functions of that preposition, but would "to" also work? Can "to" also be used in the same way; e.g. "to him, this is very important".</p>
<p>This might be a complicated question, because I've looked online and can't find a concrete dividing line between "to" and "for", especially in this case.</p>