Final answer:
The sentence fragment is 'scientist say it is mostly physical attraction,' lacking the subject 'Scientists' and the article 'the'.
Step-by-step explanation:
The sentence 'scientist say it is mostly physical attraction' contains a sentence fragment. This is because it lacks a subject to perform the action described in the sentence; the correct subject would be 'Scientists'. Additionally, it should include the article 'the' before 'scientist' to be grammatically correct. A complete sentence might read, 'Scientists say that it is mostly physical attraction.'