Spanish is the main language of Mexico (C), the world's largest Spanish-speaking nation with over 130 million speakers. Mexico's official language is Spanish and it's the primary language for daily life, government, and media. Russia (A) speaks Russian, Africa (B) is a continent with diverse languages (Arabic, French, English, Portuguese, indigenous languages), and Brazil (D) speaks Portuguese. The question asks for 'main language' not 'where it's spoken.'