Question

which is a sentence fragment? well need a detour around the flooded road heres an alternate route. has developed resistance to the drug prescribed by the doctor.

Explanation:

A sentence fragment is a group of words that does not express a complete thought. The first option "We'll need a detour around the flooded road here's an alternate route." is a run - on sentence (two independent clauses joined without proper punctuation), not a fragment. The second option "Has developed resistance to the drug prescribed by the doctor." lacks a subject, so it is a sentence fragment.

Answer:

"Has developed resistance to the drug prescribed by the doctor."