Final answer:
The Transatlantic slave trade does not belong in the list with the encomienda system, mission system, and conquistadors, as it was a broad trading system involving multiple European powers, whereas the others are directly related to Spanish colonial practices.
Step-by-step explanation:
Out of the options given (a. Encomienda, b. Mission system, c. Transatlantic slave trade, d. Conquistadors), the one that doesn't belong is c. Transatlantic slave trade. The encomienda system, mission system, and conquistadors were all direct components of Spanish colonial practices in the Americas. The encomienda was a labor system where the Spanish crown entrusted colonists with land and native inhabitants to use as forced labor, largely to work in mines or on plantations, under the pretense of providing them with Christian teachings.
The mission system was a network of missions established by the Spaniards with the intent of converting and educating the indigenous peoples in Christianity, but also served as a means of colonization. Conquistadors were the Spanish explorers and soldiers who conquered territories in the Americas. On the other hand, the Transatlantic slave trade was a broader system of trading enslaved Africans across the Atlantic by various European powers, including the Spanish, to work in the New World, but it was not specific to the Spanish and involved multiple European countries.