All of the following was used as evidence by harry hess to conclude that the sea floor was spreading, except:
question 2 options:
rocks near the mid-atlantic ridge were younger than rocks further out
rocks near the mid-atlantic ridge were less dense than those further out
sonar technology confirmed that the crust was moving
the rocks' magnetic alignments matched up with the changes in the magnetic field

The rock's magnetic alignments matched up with the changes in the magnetic field

Explanation:

Harry Hess was in many ways the successor of Alfred Wegener when it came to proving that the seafloor spreading is real and that it is one triggers the the movement of the tectonic plates. Hess was part of the Navy, so he was able to use this to map the ocean floor.

Having access to the new sonar technology, Hess managed to provide evidence that the seafloor is actually shallower in its central part, around the mid-ocean ridge, while it is the deepest close to the continental margins. Also, he managed to provide evidence that the seafloor has lower density at the mid-ocean ridge, and that the density increase as the distance increases from the mid-ocean ridge. The rocks too were younger at the mid-ocean ridge, which is understandable considering that it is a place that has active volcanism, while the places further away had older rocks.

Hess knew that he needs numerous, hard evidence in order to present his work to the scientific community and have it accepted. He was aware that if he has only one flaw or uncertainty he will be dismissed as Alfred Wegener was before him.