Question

Which type of farming should be done on slopes?

Answer 1

terrace farming is done on slopes

Answer 2

Hillside farming can be done on slopes although it requires more work to do farming on these surface.  

You will need a good soil to grow good crops. There will be high chance of rain from you work on slopes or hillside and your crops can be destroyed.

It totally depends on the land quality and slope of the land.