The best answer that would point to a sentence that has a misplaced modifier would be letter a. The phrase 'swimming away' was pertaining to the ducks. However, the modifier seemed like it was referring to the dog instead of the ducks.
The correct comparative to the adverb healthfully is more healthfully, and the superlative is most healthfully. As has been pointed out admirably elsewhere in this post, healthfully is not a commonly used adverb. However, that does not make it incorrect to use it.
1. plot 2. introduction 3. climax 4. climax 5. rising action 6. falling action 7. rising action 8. exposition