The defeat of the Aztec and Incan empires primarily enriched the Spanish monarchy through the following mechanism:
The monarchy received a portion of all treasures taken by the conquistadors.
After the conquests, Spanish conquistadors, such as Hernán Cortés and Francisco Pizarro, plundered vast amounts of gold, silver, and other valuable resources from the Aztec and Incan empires. The Spanish crown claimed a significant portion of these riches, significantly enriching the monarchy and funding further exploration and military ventures. This influx of wealth helped establish Spain as a dominant power in Europe during the 16th century.
While the other options listed, such as heavy taxation on colonies, the selling of lands, and control of the slave trade, contributed to Spain's wealth, the immediate and direct benefit from the conquests of the Aztecs and Incas was the treasure obtained and claimed by the crown.