Exploration and Description (B): This is typically the first step in the study of human geography. It involves the initial exploration and description of geographical phenomena, often in the form of fieldwork or observational studies. 

Regional Analysis (D): Regional analysis builds upon areal differentiation and goes deeper into the study of specific regions. Geographers examine the spatial patterns, processes, and interactions that occur within these regions. They may investigate topics such as regional economic development, cultural landscapes, or environmental issues. 

Areal Differentiation (A): Once the exploration and description phase is complete, the focus shifts to areal differentiation. This involves the classification and differentiation of geographical areas based on specific characteristics or criteria. 

Spatial Organization (C): Spatial organization is the highest level of analysis among the options you've provided. It focuses on understanding how various elements are arranged and organized in geographical space. This approach delves into the broader spatial relationships and structures that exist at a larger scale.

So, option 3 is correct.