Explanation
Correct answer: D.
Choice D is the best answer because Text 2 presents a possible solution to the problem identified in Text 1—that is, how disparities in the criteria used by different countries to define urban settlements make it "difficult for researchers to compare aspects of urbanization in different countries." Text 2 describes the creation of a shared framework for studying global urbanization: "global definitions of common types of settlements" and a "new measure called the ‘degree of urbanization’" that "allows researchers to better understand global urbanization rates."
Why the other choices are wrong
Choice A
Choice A is incorrect. Although Text 2 mentions "a group of six international organizations" that developed criteria and a measure related to settlements and urbanization, no organization or institution is singled out to perform further research.
Choice B
Choice B is incorrect because both texts are about urbanization research and neither mentions any other fields of study.
Choice C
Choice C is incorrect. Text 2 discusses a possible solution to the terminological problem in global urbanization research, not additional areas of difficulty in the field.