What is the correct reason for thinner crust below oceans?

Correct answer: Oceanic crust is mainly basaltic and keeps forming anew

Oceanic crust is thinner than continental crust mainly because it has a different composition and geological history. It is formed largely from basaltic magma at mid-oceanic ridges, where new seafloor is continuously created. Oceanic crust is generally about 5–10 km thick, whereas continental crust is much thicker and is commonly around 30–70 km thick. The repeated formation, movement and eventual subduction of oceanic crust help maintain this relatively thin layer. Its basaltic composition and its renewal at spreading ridges make option A the best available answer.

The core is not absent beneath oceans; Earth’s core lies below the mantle everywhere. Seawater cannot reduce crust to zero, and oceanic crust does not float in the atmosphere. The key comparison is between the thin, dense basaltic oceanic layer and the thicker continental layer. Strictly speaking, new formation alone is not the only explanation for its thickness, but among the given choices option A correctly identifies the important basaltic nature and continuing creation of oceanic crust. Therefore, A is acceptable.