Final answer:
Mazatlán, being just south of the Tropic of Cancer and described as the largest Pacific port city in the question, is located in Mexico.
Step-by-step explanation:
The city of Mazatlán is located just south of the Tropic of Cancer and is known as the largest Pacific port in the country it resides in. Considering the geographic location and the fact that Mazatlán is mentioned as a port city, the correct answer is A) Mexico. This answer is supported by the knowledge that Mazatlán is indeed a major city on the Pacific coast of Mexico. Additionally, the direct reference to the Tropic of Cancer tells us that the city and country in question are north of the equator and southern boundary of countries like Chile and Argentina, further confirming that the answer is Mexico.