Fill in the blank. when the northern hemisphere experiences autumn, the southern hemisphere experiences a spring b summer c autumn d winter

Your answer is Spring.

The Earth is on a tilt of 23.5 degrees, leading for different weather in different hemispheres, though, when the northern hemisphere experiences autumn, the southern hemisphere experiences spring. So, while the northern hemisphere is having winter during months such as January, the southern hemisphere would be having winter during June, and so on. They are always opposite seasons.