Which layer is most observed and studied by scientists

The correct answer is - the crust.

Explanation:

The crust is the outermost layer of the earth made up of silica and oxygen mainly. The crust is the most studied and observed layer of the earth as it is the first layer.

The crust can be divided into the continental crust (32 km thick under) and oceanic crust (8 km thick under). It is the thinnest layer of the earth that makes the lithosphere with the upper layer of the mantle with it.