Which observation provides direct evidence for seafloor spreading as part of plate tectonics?

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The observation that directly shows seafloor spreading is the pattern recorded in the ocean floor rocks where new crust forms at mid-ocean ridges and then moves outward. As magma rises and solidifies at these ridges, it records the Earth's magnetic field. Over time, the field has flipped many times, so the rocks on either side of a ridge show mirror-image magnetic stripes that align with the history of reversals. At the same time, the youngest rocks are found right at the ridges and get progressively older with distance away from them. This combination—magnetic stripes plus age progression of the seafloor—directly demonstrates that crust is being created at ridges and pushed outward, which is the essence of seafloor spreading within plate tectonics. The other options involve processes unrelated to how the seafloor moves and forms, such as lunar phases, atmospheric patterns, or ocean salinity, so they don’t provide direct evidence for seafloor spreading.