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. According to this theory, mid - ocean ridges are the sites where new oceanic crust is formed through the upwelling of magma, which is the process of seafloor spreading. Continents do move (contrary to the first option). It is hot rock (magma) that rises, not cool rock (rejecting the second option). Trenches are sites of subduction, not seafloor spreading (rejecting the third option).

Answer:

Mid - ocean ridges are sites of seafloor spreading.