<p>(Along) the curve of islands known as the Florida Keys (lies) a reef of living coral, (the only one) of (a kind) in the continental United States. (No Error)</p>
<p>**lets assume the parentheses are underlined
the answer is D (a kind) why is that wrong?</p>
<p>“one of a kind” would not make the sentence unidiomatic or redundant</p>
<p>“one of a kind” means “the only one of a particular type”
“one of its kind” means “one of its type,” or “one of this particular type”
“only one of its kind” means “only one of its type,” or “only one of this particular type”</p>
<p>“one of a kind” is often used as an adjective: i.e., he’s one of a kind … or … one-of-a-kind coral reef …</p>
<p>On the other hand, “one of its kind” is “one of its type” …</p>
<p>The word “only” in front of “one of a kind” makes it awkward (or redundant as some people pointed out), but “only one of its kind” is used all over the places (search for “one of its kind”):</p>