A is the only answer that would actually make sense logically. Because an illusion isn't actually real, but it always represents something one way or another.<span />
Allusion is a reference to another piece of literature or published work. In literature there are many references to biblical stories or characters, since it is a widely read text. Other common allusions are references to famous couples like Bonnie and Clyde or Romeo and Juliet. In order for a reader to understand the allusion, he must understand what the allusion represents.
