Answer:
A
Step-by-step explanation:
An allusion is a reference to something without mentioning it. If you were to tell someone, "This new war is Vietnam all over again," like the explanation offers, the reader would have to know that a war happened in Vietnam in order to understand that reference. The writer also didn't outright state that a war happened in Vietnam, fitting in to the criteria of what makes an allusion an allusion.
The rest of the choices aren't subtle enough, D isn't referencing anything really.