Answer:
4. reliable rainfall
Step-by-step explanation:
Reliable rainfall is important to obtain water source.
Mayan and Olmecs heavily rely on agriculture to sustain their lives and as one of the biggest contributor to their economy. (they often trade their agricultural products to other groups)
The rainfall allow them to build agricultural farm that is not necessarily close to other water sources such as lake or river. This will increase the agricultural goods that they can produce each year.