Answer: B: Siddhartha Gautama gained insight into human suffering.
Explanation: These are the events listed IN ORDER:
1- Siddhartha Gautama became disenchanted (unhappy) with life.
2- Siddhartha Gautama wandered through the forests for years.
3- Siddhartha Gautama meditated underneath a tree.
4- Siddhartha Gautama gained insight into human suffering.
I hoped this helped! :)