The portion of the continental margin that serves as a boundary between oceanic crust and continental crust is the continental slope.

Here's why:

* Continental Shelf: This is the relatively shallow, gently sloping area that extends from the shoreline out to the continental slope. It's part of the continental crust.

* Continental Slope: This is the steep drop-off that marks the transition from the shallow continental shelf to the deep ocean floor. It's where the continental crust transitions to oceanic crust.

* Continental Rise: This is the gently sloping area at the base of the continental slope. It's formed by sediments that have been transported down the slope and deposited.

Therefore, the continental slope is the key boundary between the continental crust and oceanic crust.