Question

which is a sentence fragment? the maps legend indicates the meaning of its symbols. the undercover agent still investigating that strange case. the veins in our bodies return blood to the heart.

Explanation:

A sentence fragment lacks a complete thought. The first and third options are complete sentences with a subject and predicate. The second option "The undercover agent still investigating that strange case" is missing a verb (e.g., "is" before "investigating") to form a complete predicate, making it a fragment.

Answer:

The undercover agent still investigating that strange case.