The correct statement is: Mexico is the largest Spanish-speaking country in the world.
To clarify the other options:
- The Aztec people spoke Nahuatl, not Spanish.
- Spanish is not the native language of the Mayans; the Mayans spoke various Mayan languages.
- Nahuatl is not the basis of the Spanish language; Spanish is a Romance language that evolved from Latin. However, Nahuatl has contributed some words to the Spanish language.