Which one of the following is not a tropical desert?

Deserts are classified geographically by climate/latitude: a TROPICAL (hot) desert lies within roughly 15°-30° latitude, in the belt of subtropical high pressure, and so stays hot through the year; a COLD or TEMPERATE desert lies at higher, continental-interior latitudes and has freezing winters despite low rainfall.

Checking each option by its latitude and climate:

• Arabian Desert — on the Arabian Peninsula, roughly 15°-30°N, squarely in the subtropical high-pressure belt: a tropical (hot) desert.

• Kalahari Desert — Southern Africa, roughly 20°-28°S, dried by the same subtropical high pressure: a tropical (hot) desert.

• Atacama Desert — coastal Chile/Peru, roughly 20°-30°S; its extreme dryness comes from the cold Humboldt current and a rain-shadow, but its latitude still places it among tropical/subtropical deserts.

• Gobi Desert — Mongolia and northern China, roughly 40°-50°N, deep in the continental interior of Asia, shielded from monsoon moisture by the Himalaya-Tibetan Plateau mountain system, with freezing winters: a cold/temperate desert, outside the tropical belt.

Cross-check: the Tropic of Cancer (23.5°N) runs through the Arabian Peninsula, and the Tropic of Capricorn (23.5°S) runs through the Kalahari region (Botswana) and the Atacama (northern Chile) -- placing all three within or at the very edge of the tropics. The Gobi, at 40°-50°N, lies well outside the tropics altogether, confirming it alone is non-tropical.

So among the given options, the Gobi is the one that is not a tropical desert.