Carl is on an expedition to the bottom of the ocean. As the submersible sets down on the ocean floor, it is resting on what layer of the earth?

Carl is still on the outermost layer of the Earth - he would have to drill through it to get to the inner layers (mantle and core). This layer is called the Crust and this is the correct answer.

You can also be more specific and say that Carl is on the Oceanic, not continental Crust.