Question

which concept is described in the theory proposed by hess?
continents do not move.
cool rock pushes through the crust.
trenches are sites of seafloor spreading.
mid - ocean ridges are sites of seafloor spreading.

Explanation:

Hess proposed the theory of seafloor - spreading. Mid - ocean ridges are the key sites where new oceanic crust is formed through seafloor spreading. Continents do move (contrary to the first option). It is hot, not cool, rock (magma) that rises through the crust. Trenches are sites of subduction, not seafloor spreading.

Answer:

Mid - ocean ridges are sites of seafloor spreading.