Most of what we know about the Olmec civilization comes from A.
their books and writings.
B.
the stories of Aztec historians.
C.
the diaries of Spanish explorers.
D.
their sculptures and artifacts.

Their sculptures and artifacts.

Explanation:

The Olmecs were extremely talented artists and sculptors: they produced many statues, masks, figurines (Yes, figurines were a thing back then), stelae (I know you're confused. A stela (the singular form of stelae) is an upright stone or column typically bearing a commemorative inscription or relief design, often serving as a gravestone.), thrones and more. However, they are best known for their colossal heads, 17 of which have been found at four different archaeological sites.

The first picture is an Olmec head, and the second is a stela.

north atlantic treaty organization.

i think it's b