The most grammatically correct sentence without fragments is:
"Daydreaming about his trip to Hawaii, Zach couldn't concentrate on his exam."
Explanation:
1. This sentence is a complete sentence with a subject (Zach) and a predicate (couldn't concentrate on his exam).
2. The introductory phrase "Daydreaming about his trip to Hawaii" is followed by a comma, which correctly sets it off from the main clause.
3. The structure of the sentence is clear and follows the standard subject-verb-object order, making it grammatically correct.
