The correct answer is "the desire of wise men".
In her play "Frankenstein" (1823), Mary Shelley uses different syntactic strategies in order to put emphasis on certain information. For example, she could have written something like this: "within my grasp is the study and desire of the wisest men...".
Instead, she chose to present the information in a rhetorical way. She introduced the "wh" word "what" opening the window for the reader to question himself what is the meaning of "what"?
What is THAT THING that had been the study and desire of wisest men since the creation of the world? What did they want to know?
