Final answer:
Acheulean tools are characterized by their sophisticated, symmetrical teardrop shape achieved through the soft hammer technique, which was an advancement over the hard hammer percussion method used to create Oldowan tools.
Step-by-step explanation:
The main characteristic of the more complex and diverse stone-tool kit described as "Acheulean tools" is that these tools were much more sophisticated than the earlier Oldowan tools. Acheulean hand-axes are distinct in that they were bifacially modified, which means they were chipped and formed on both sides to create a symmetrical and teardrop-shaped implement, providing a sharper and more efficient cutting edge. The process that produced these tools involved the soft hammer technique, where softer materials like wood or bone were used to chip away smaller flakes from a stone core, resulting in a smoother and more uniform surface compared to the hard hammer percussion method used for Oldowan tools.