The answer to the question regarding how many ducks it would take to eat a pie is letter C or I don’t know (IDK). It is most probably because nobody knows if a duck can eat a pie and if it can, how many it would take for them to eat a pie. However, ducks eat a variety of food such as fish eggs, small fish, worms, grass, weeds, small frogs, , salamanders, small berries, etc.
Although answering this question using the given variety of food previously enumerated would defeat the purpose of this joke. I provided some examples of the food they eat just in case you want to really know.