Final answer:
Among the given options, making a snowball is a non-spontaneous event because it requires external energy input, unlike the other options which occur spontaneously. So the correct answer is Option D.
Step-by-step explanation:
The question asks which of the following describes a non-spontaneous reaction/event. A non-spontaneous process is one that does not occur naturally under given conditions without the continual input of energy from an external source. Among the options given:
- Leaves falling off a tree
- A balloon popping
- Water flowing downstream
- Making a snowball
All of these events generally occur spontaneously, except for making a snowball, which requires an input of energy from an external source, like a person forming the snowball with their hands. This process does not occur without some form of energy input, thus it is a non-spontaneous event.