The correct option is The upper portion of the mantle extending up to 400 km..
Explanation
The Earth's interior is stratified into layers based on chemical composition (Crust, Mantle, Core) and mechanical properties (Lithosphere, Asthenosphere, Mesosphere, Core). The Asthenosphere represents a specific mechanical zone within the upper mantle.
Option Analysis:
- The solid lower mantle. is incorrect: The solid lower mantle is known as the Mesosphere. Despite high temperatures, the immense pressure keeps this layer rigid.
- The liquid outer core. is incorrect: The liquid layer of the Earth is the Outer Core, which is composed primarily of molten iron and nickel.
- The upper portion of the mantle extending up to 400 km. is correct: The Asthenosphere (derived from the Greek word asthenes, meaning weak) is the upper portion of the mantle extending up to approximately 400 km. It is characterized by its low rigidity and plasticity (semi-molten state). This ductility allows the overlying lithospheric plates to move. It is also the main source of magma for volcanic eruptions.
- The crust and the uppermost solid mantle. is incorrect: The combination of the crust and the uppermost solid mantle is called the Lithosphere. The Lithosphere is a rigid slab that floats over the ductile Asthenosphere.
Key Takeaway: The Asthenosphere is the highly viscous, mechanically weak, and ductile region of the upper mantle (below the Lithosphere) that facilitates plate tectonic movement and magma generation.