Which will most likely form when movement along a plate boundary forces a landmass to be pulled apart?

When a landmass is pulled apart due to movement along a plate boundary, a continental rift is most likely to form. This process occurs at divergent boundaries, where tectonic plates move away from each other. As these plates separate, the land in between can begin to sink and fracture, leading to the formation of a rift valley. This rifting can create features such as cracks in the Earth's crust and, over time, may lead to the development of a new ocean basin if the rifting continues.

In contrast, volcanic island arcs typically form at convergent plate boundaries where an oceanic plate subducts beneath another plate, resulting in volcanic activity. Continental mountains are generally associated with the collision of tectonic plates, leading to uplift rather than extension. Oceanic trenches also arise from processes at convergent boundaries, particularly where one tectonic plate descends below another. The mechanism that causes rifting and the associated geological phenomena underscores why a continental rift is the most appropriate answer in this case.