Final answer:
The study of landmarks in downtown Houston pertains to the geography theme of 'Place', which focuses on the unique human and physical characteristics of a location.
Step-by-step explanation:
The study of landmarks in downtown Houston would address the theme of Place among the five themes of geography. The theme of Place relates to the human and physical characteristics that make a location unique. In the case of downtown Houston, landmarks are distinctive features that define the character and identity of the city's central area. These landmarks can have political, economic, social, and cultural significance, thus contributing to a place's unique characteristics. This theme captures the essence of what makes a location special or notable, as opposed to solely its geographic coordinates or the movements and interactions that occur there.