D? - slash-and-burn agriculture is best defined as the process of cutting plants and setting fires to fertilize farmland for crops. cutting steps into a hillside to make a flat surface for crops. using ash from fires to make farmland fertile for decades. clearing forests to make farmland fertile for generations.

The correct answer is cutting plants and setting fires to fertilize farmland for crops. This was common historically where you would set forests or fields on fire to burn them down and then you'd have a land big enough for crops. This was quite common in times where there was no technology, but now its use is rare.