Which type of sentence is this?
Annual plants survive only one season and must be replaced each year, but perennials return each year with new flowers and foliage.

 - complex
 - simple
 - compound-complex
 - compound
The sentence is compound. It is made up of two independent clauses joined by the coordinating conjunction but.
Annual plants survive only one season and must be replaced each year, but perennials return each year with new flowers and foliage.