The correct answer is C. they both farmed corn.
Both the Maya and the Olmec societies relied heavily on agriculture, with corn (maize) being a staple crop that was essential to their economies. This agricultural continuity played a crucial role in their societies, providing a reliable food source that supported population growth and stability. While both cultures engaged in trade and utilized various materials and goods, corn was fundamental to their economic systems and cultural practices.
Options A and B are not as accurate since the concept of a "money system" may not have been formalized in the way we consider currency today, and while limestone was utilized, it wasn't a primary trade good or the backbone of their economies. Option D, while cacao was important, particularly for the Maya, it wasn't as universally foundational as corn.