Final answer:
The plate tectonics theory states that the deepest portion of the rigid lithospheric crustal plates is formed from mantle material.
Step-by-step explanation:
The theory that states the deepest portion of the rigid lithospheric crustal plates is formed from mantle material is the plate tectonics theory. According to this theory, the lithosphere, which includes the crust and the uppermost rigid portion of the mantle, is composed of tectonic plates that move and interact with each other.
These plates are driven by the slow convection of the mantle, which causes the movement and separation of the plates. The mantle material is responsible for forming the deepest portion of the crustal plates through processes like subduction and volcanic activity.