Question

drag the missing word into place
the earth is tilted on its axis,
so its surface is unevenly heated by the sun.
this is why some regions stay warmer than others.
there are areas of warm and cool on the earth.

Explanation:

The text is about the Earth's uneven heating due to its axial tilt. The missing word should relate to the medium (air) that is heated unevenly. "Air" makes sense as it is the substance that gets warmed (in warm areas) and cooled (in cool areas) based on the sun's heating. "Equator" is a specific line on Earth and doesn't fit the context of "areas of warm and cool...". "Convection" is a process (like heat transfer by fluid movement) and not the thing that has warm and cool areas.

Answer:

air