Question

which is a sentence fragment? is an expert at distinguishing safe mushrooms from poisonous ones. two hydrogen atoms and one oxygen atom compose each molecule of water.

Explanation:

A sentence fragment lacks a subject or a verb or does not express a complete thought. The first option "Is an expert at distinguishing safe mushrooms from poisonous ones." has no subject. The second option is a complete - thought sentence.

Answer:

Is an expert at distinguishing safe mushrooms from poisonous ones.