Which of the following is a simple sentence?

A simple sentence contains one independent clause only, with a subject and verb. Note that the long compound phrase "with the shiny blue collar and the white paws" is a prepositional phrase describing the subject, "cat," and "to my great-aunt Ellie" is another prepositional phrase describing the belonging.
"Wherever we go to find happiness." is not a complete sentence since it does not express a complete thought.
"While Edgar painted, Fran sanded the floors." is a complex sentence, containing one dependent and one independent clause.
"Medical knowledge is important, but sympathy is also important." is a compound sentence made up of two independent clauses joined with a coordinating conjunction.