The answer is A. themes; subfields
This option correctly identifies that the "five themes" are tools for studying any topic in geography, such as location, place, human-environment interaction, movement, and region. The "five subfields" refer to the areas geographers study, which include physical geography, human geography, environmental geography, geographic information systems (GIS), and regional geography.
This option incorrectly suggests that "subfields" are tools for studying any topic, which is not accurate. Subfields are specific areas within geography, not tools.
This option reverses the correct order, suggesting that subfields are tools and themes are what geographers study, which is incorrect.
This option incorrectly suggests that "regions" are tools for studying any topic, which is not accurate. Regions are a theme within geography, not a tool.