The correct answer is option 'D': B, D, C, A.

1. Exploration and description:
- This approach to studying human geography was the earliest one, dating back to the ancient civilizations. Scholars and explorers traveled to different regions, observed the physical and cultural characteristics of those areas, and documented their findings. It involved collecting data through fieldwork, mapping, and describing the natural and human features of different places.

2. Regional analysis:
- The regional analysis approach emerged in the late 19th and early 20th centuries. Geographers started to focus on studying regions as distinct units with their own characteristics, rather than just describing individual places. They analyzed the spatial patterns and relationships within regions, examining the physical, social, economic, and cultural aspects. This approach aimed to understand the unique qualities of different regions and how they interacted with each other.

3. Areal differentiation:
- Areal differentiation is a concept that emerged in the mid-20th century. It focuses on understanding the spatial variations in human geography. Geographers started to explore the differences and similarities between regions, looking at factors such as population density, economic activities, cultural practices, and environmental conditions. This approach aims to identify patterns, trends, and relationships across different areas.

4. Emergence of humanistic school:
- The humanistic school of thought in human geography emerged in the late 20th century. It shifted the focus from the physical and economic aspects to human experiences, perceptions, and meanings attached to places. Geographers studying human geography from a humanistic perspective analyze how individuals and communities interact with their environment, how they perceive and give meaning to places, and how their experiences shape their identities and behaviors. This approach emphasizes the importance of subjective experiences, cultural values, and individual agency in shaping human geography.

In summary, the chronological order of the approaches to study human geography is: Exploration and description, Regional analysis, Areal differentiation, and Emergence of humanistic school.