The climatic zones are classified on the basis of various factors. Option 1 suggests that the distance from the equator plays a role in defining climate zones. This is true as the distance from the equator affects the amount of sunlight and temperature experienced in an area. Option 2 proposes that elevation determines climatic zones. While elevation does have an impact on climate, it is not the sole factor defining climatic zones. Option 3 states that rainfall is the basis for classifying climatic zones. This is correct as the amount of precipitation greatly influences the type of vegetation and overall climate of an area. Option 4 suggests that proximity to the sea determines climatic zones. While it is true that coastal areas may have a distinct climate influenced by the sea, this is not the only factor considered when classifying climatic zones. Therefore, the correct answer is option 3 as it accurately identifies rainfall as a crucial factor in defining climatic zones.