What layer of the Earth is less than 50 km thick over the oceans?

The crust is the correct answer because it is the outermost layer of the Earth and varies in thickness depending on whether it's beneath oceans or continents. Over the oceans, the oceanic crust typically ranges from about 5 to 10 km in thickness, which is indeed less than 50 km. This thin layer is primarily composed of basaltic rock, which is denser than the continental crust but significantly thinner.

In contrast, the mantle is much thicker, extending to a depth of about 2,900 km and is not applicable here since it lies beneath the crust. The core, consisting of a solid inner core and a liquid outer core, is also much thicker than 50 km, making it irrelevant to this question. The term "astratosphere" appears to be an incorrect term related to atmospheric science rather than geology and does not pertain to the layers of the Earth.