Question

the mantle is a region of semi - solid

Explanation:

In Earth science (a subfield of Natural Science, specifically related to Geography or Geology), the mantle is composed of semi - solid rock (magma - like material in a semi - solid state, often described as having a semi - solid or plastic - like consistency, with the main component being rock). The most appropriate term to fill in the blank is "rock" (or more specifically, "magma" can also be used, but "rock" in a semi - solid state is a common description for the mantle's material).

Answer:

rock (or magma, with "rock" being a more general and commonly used term in this context for the semi - solid material of the mantle)