Question

what property was used as evidence to support seafloor spreading?
○ density
○ boiling point
○ sonar
○ magnetism

Explanation:

Seafloor spreading was supported by observing magnetic striping on the ocean floor. As new seafloor forms at mid-ocean ridges, it records Earth's alternating magnetic field, creating symmetric magnetic patterns on either side of the ridges. The other options are irrelevant: density and boiling point are not related to this geological evidence, and sonar was used to map the seafloor, not as direct evidence for spreading itself.

Answer:

magnetism