Marlena, who enjoys many different subjects, likes history and geography the best. Which part of the sentence is the nonrestrictive modifier?

A non-restrictive modifier add information that does not restrict who the subject can refer to (compare to "The one Marlena that enjoys many subjects" - this one restricts the subject to a specific Marlena). Here it's "who enjoys many different subjects" - it adds additional information about her, but doesn't change the referent.