Explanation:

Soil fertility is influenced by several natural factors, including parent material, climate, organisms, topography, and time (soil age). Parent material (Option C) determines the initial mineral composition. Topography (Option D) affects drainage, erosion, and microclimate. Soil age (Option A) refers to the duration of soil formation processes, impacting nutrient cycling and profile development. While air (specifically soil aeration) is crucial for microbial activity and root respiration, which are vital for fertility, 'Air' itself is not considered a primary *differentiating natural factor* that *determines* the inherent fertility potential of a soil type in the same way as the other options. Air is a component of the soil environment, but its presence and composition are largely influenced by the other natural factors and soil structure. Therefore, among the given choices, 'Air' is the least fitting as a distinct, primary natural factor affecting soil fertility in terms of its inherent characteristics.