Final answer:
The correct match is 'a. mid-ocean ridge / sea-floor spreading' as sea-floor spreading is a process that occurs at mid-ocean ridges, where tectonic plates diverge and new oceanic crust is formed by molten rock rising to fill the gap between the plates.
Step-by-step explanation:
This is because sea-floor spreading is the type of process that happens at a mid-ocean ridge, where tectonic plates move apart from each other, allowing molten rock to rise from below to fill the space between the receding plates, forming new oceanic crust.
At divergent boundaries, such as a mid-ocean ridge, plates move apart and new crust is created by upwelling molten rock that cools to form new basaltic oceanic crust. An example of sea-floor spreading can be seen at the Mid-Atlantic Ridge. The other pairs in the question do not correctly match a geologic feature with the corresponding tectonic process. For instance, at convergent boundaries, plates move towards each other, not apart, and at transform boundaries, plates slide past one another.
Furthermore, subduction is a process associated with convergent boundaries, not mid-ocean ridges. During subduction, one tectonic plate is forced beneath another into the Earth's mantle, which often leads to the formation of a trench and volcanic activity, but is not directly related to the constructive processes at mid-ocean ridges.