My first thought was the same as number 3, the scientific method. But looking at history, many important discoveries were made long before the scientific method

My first thought was the same as number 3, the scientific method. But looking at history, many important discoveries were made long before the scientific method was formalised. It seems some weak, intuitive form of the scientific method came naturally to many people throughout history. So I would rather go with something that took a very long time to be understood and was absolutely necessary to create any civilization. For example number 4, basic hygiene. Or agriculture: "you can grow edible plants on purpose by planting their seeds in the ground and watering them". The scientific method is not very useful if you have to spend your life hunting and gathering to survive.