Which group of words is a sentence rather than a fragment? work their way a few centimeters into the soil and feed on the sap of tree roots. by fastening themselves securely to tree trunks to molt their outer cases. cicadas live only one month. trying to stay out of sight, move around a branch, hiding from view?

The correct answer is Cicadas live only one month.

This is the only group of words which can function as a sentence rather than a fragment. A fragment is a group of words which cannot exist on its own - they have to be included into a bigger sentence. On the other hand, independent clauses (such as the correct answer in this example) can exist on their own.