The landform that develops at plate boundaries in the ocean where magma rises and melts is a mid-ocean ridge.

Here's why:

* Plate Boundaries: Mid-ocean ridges occur at divergent plate boundaries, where tectonic plates move apart.

* Magma Rising: As the plates pull away, the space created allows magma from the Earth's mantle to rise and erupt.

* Melting: The pressure decrease as the magma rises causes it to melt, contributing to the volcanic activity.

* Ridge Formation: The solidified lava accumulates along the boundary, forming a long, underwater mountain range called a mid-ocean ridge.

So, the combination of divergent plate movement, magma rising, and subsequent volcanic activity creates the unique landform of a mid-ocean ridge.