Final answer:
Skin color is mainly correlated with latitude because UV radiation levels vary with latitude, affecting the evolutionary skin color adaptations in humans. Temperature, humidity, and altitude influence environmental conditions, but are not as directly related to skin color.
Step-by-step explanation:
Skin reflectance, or skin color, is an adaptive trait that is correlated with various geographic and environmental factors. Of the options listed, research has found a strong correlation with latitude. Historically, populations that lived closer to the equator developed darker skin, likely as a protective measure against higher levels of ultraviolet (UV) radiation. Those living at higher latitudes, where UV radiation is less intense, generally have lighter skin. This pattern is illustrated in geographical maps showing skin color predictions based on UV radiation levels.
While temperature, humidity, and altitude also have roles in environmental conditions, they are not as directly correlated with skin color as latitude. Skin color adaptation is a response to UV radiation, which is strongly influenced by the angle of the sun's rays that changes with latitude.
When considering how much energy enters Earth's system, the term albedo is used to describe the reflectivity of a surface. Earth's overall albedo affects global temperature, and changes in surface reflectivity, such as the melting of polar ice caps, can have significant impacts on global climate systems.