Answer:The guests at the picnic decided not to eat the pie covered with ants.
Step-by-step explanation:
This is the best way to write the sentence because at first it is showing us the a noun(the guests) which are decided(verb) to do something.
In the first sentence we can get a feeling like the guests are covered with ants and not the pie. Same works for the second and the last sentence.
The third sentence is one making the most sense.