The choice that best describes the problem for the sentence "Hoping for a warm day, the picnic supplies were all packed." is:
dangling modifier
This is because "Hoping for a warm day" is a participial phrase that appears to modify the subject that immediately follows it, which in this sentence is "the picnic supplies." However, picnic supplies cannot hope for a warm day; a person does that. The sentence lacks a proper subject to which the modifier can logically attach, leaving the modifier dangling.