Step 1: Understand the term "agglomeration economies" from the passage. The passage describes agglomeration economies as differing across habitations, with examples like scale economies (market towns), localisation economies (medium-sized cities), urbanisation economies (larger cities), and a combination of these in metropolitan regions.
Step 2: Analyze the characteristics. It highlights that metropolitan regions have "jointly-generated and mutually-reinforcing agglomeration economies," indicating a clustering of various economic activities (localisation, urbanisation, networking) that work together to boost productivity and innovation.
Step 3: Match with the options. Option A (national economies) is too broad, option B (a differentiation of different economies) focuses on differences rather than clustering, and option C (the agricultural economy) is too narrow. Option D (a lot of different economies gathered together) best captures the idea of diverse economic activities clustering and reinforcing each other, as described in the passage. Thus, the correct answer is D.