Answer: but.
Explanation: a preposition is a word governing, and usually preceding, a noun or pronoun and expressing a relation to another word or element in the clause. Some examples of prepositions are to, of, by, between, under, through, near, etc. In the given sentence we can see an example of a preposition in the word "but" preceding the proper noun "Elsa" which is the object of the preposition.
