The correct answer is C. Illusion.
A genuine item is one that is real and not fake. An authentic item is one that is genuine and true to its original form. A mirage is an optical illusion that makes something appear to be where it is not. An illusion is a false or misleading perception of something.
In the original pair of words, "genuine" and "authentic" are synonyms. They both mean "real" or "true." In the answer pair, "mirage" and "illusion" are also synonyms. They both mean "false" or "misleading."
The other options are not synonyms of each other. A reflection is an image that is formed when light bounces off of a surface. A hideout is a place where someone can hide. An image is a representation of something.