Final answer:
The term 'nexus' rhymes with 'Texas' and refers to the core of something, such as Mexico City in Mexico or the megalopolis from Washington, D.C., to Boston in the United States.
Step-by-step explanation:
The term that describes the center or core of something, and rhymes with Texas, is A) Nexus. In a broader context, the term 'core' often refers to the economic, political, or cultural hub of a region. For example, Mexico City serves as the core of Mexico, being the center of activity, industry, wealth, and power. Similarly, the economic heart of the United States and Canada can be found in the megalopolis stretching from Washington, D.C., to Boston, which holds a significant concentration of the population, corporations, financial markets, and cultural institutions.