What is the upper part of the mantle known as?

The correct answer is that the upper part of the mantle is referred to as the lithosphere. The lithosphere includes both the uppermost layer of the mantle and the crust, which together form a rigid layer of the Earth's structure. It plays a critical role in geological processes and is significant for tectonic plate dynamics since these plates float on the more ductile asthenosphere located beneath the lithosphere.

The lithosphere is characterized by its rigidity and relatively low temperature, especially when compared to deeper layers, which allows it to maintain its solid state. Understanding the structure of the lithosphere is essential for comprehending various geological phenomena, including earthquakes and volcanic activity.

Other provided choices, like the asthenosphere, which lies directly beneath the lithosphere and is known for its semi-viscous properties, as well as the core and the term "mantle" in a general context, do not specifically refer to the upper layer of the mantle. Thus, they do not accurately identify the component being asked about in this question.