The great variety in the Spanish language can be primarily attributed to:
A. influence of indigenous languages
Spanish has been influenced by various indigenous languages in regions where Spanish is spoken, particularly in Latin America. This has led to significant regional variations and the incorporation of indigenous vocabulary and expressions.
While B (influence of the French and Portuguese during colonialism) may have some impact, it's not as pronounced as the influence of indigenous languages. Option C (Spanish is a really complex language) does not directly account for variety, and D (there isn't much variety in the Spanish language) is inaccurate. Therefore, the best answer is A.