Answer: D. Ginny had cut the Gordian knot by applying a formula that she had learned in Mrs. Stark's class.
Explanation:
An allusion refers to the use of a word or phrase that has a certain meaning or implication to explain a situation. For instance:<em> ''our star runner is the Flash''</em>. The implication here is that the star runner is really fast because the Flash is really fast as well.
The allusion above is the Gordian knot which was known to be so hard to untie that only Alexander succeeded and he did so by cutting it open. The sentence is therefore describing a very difficult problem that Ginny solved.
