Final answer:
The statement that is NOT true about the Olmecs is that they were the first Mesoamerican empire. They were more accurately a collection of city-states with cultural and economic influence, rather than an empire.
Step-by-step explanation:
When addressing the statements about the Olmec civilization, we must evaluate each option. The statement that is NOT true about the Olmec is: c-They were the first Mesoamerican empire. While the Olmecs were among the earliest complex societies in Mesoamerica and influenced subsequent civilizations, referring to them as an empire is misleading. They were more accurately a collection of city-states with cultural and economic influence, rather than an empire with a singular political structure dominating a vast area.
The other options listed, such as building earthen mounds grouped around a plaza (a), carving massive stone heads (b), and living along the Gulf coast of modern Mexico (d), are historically accurate and well-documented aspects of Olmec civilization.