Which of the following is considered a commercial hub of a city?

The Central Business District (CBD) is considered the commercial hub of a city because it is the area where the highest concentration of business and commercial activities occurs. The CBD typically hosts a large number of corporate offices, financial institutions, retail stores, and professional services. This concentration of economic activity attracts a significant workforce and generates a high level of foot traffic, making it a vibrant area for commerce.

In contrast, residential areas are primarily designed for living spaces and do not focus on commercial activities. Suburban zones might have some commercial establishments but are mostly characterized by residential developments and lower density compared to the CBD. The industrial sector is primarily concerned with manufacturing and production activities rather than direct commercial interactions with consumers. Therefore, the Central Business District is distinctly recognized as the commercial heart of a city, making it the correct answer.