Final answer:
One of the results of the Spanish invasion of Mexico was the substantial decrease in the indigenous population by about 20 million by the end of the sixteenth century, due primarily to the introduction of diseases like smallpox and the enforcement of colonial policies.
Step-by-step explanation:
One result of the Spanish invasion of Mexico was a dramatic decline in the country's indigenous population. By the end of the sixteenth century, 20 million people had perished, representing an unprecedented demographic catastrophe. Diseases such as smallpox, which the natives had no immunity to, played a significant role in this population decline, along with warfare and harsh conditions under Spanish colonial policies.
The correct answer to the provided question is A. Mexico’s indigenous population decreased by about 20 million people by the end of the sixteenth century.