The answer is:
Why does a duck eat?
Explanation:
If we take a look at the scientific method, the first step is observation. We need to observe things and then we move towards the formation of a hypothesis. So by looking a duck while the duck is eating, the first question that must come to our minds should be " Why does a duck eat?" And hence we can say that this is the best scientific question as it leads us towards the rest of scientific research.
