Considering that a sentence, or a full sentence, implies both a subject and a verb, the correct answer in this case would be:
Rain forests and swamps are fragile ecosystems.
Rain forests and swamps is the subject, and are fragile ecosystems is the predicate, that includes the verb.
Moreover, to draw a distinction between a full sentence and a phrase or clause, we must bear in mind that a sentence always begins with a capital letter and ends with a period or a question or exclamation mark.
