The thinnest layer of the Earth is called the crust. The crust is the relatively thin outermost layer which forms the Earth`s surface. It is made up of solid rock and is divided into two types based on the composition: continental crust and oceanic crust. The continental crust is thicker and less dense compared to the oceanic crust.

Option 1: The mantle is a layer beneath the crust and is much thicker than the crust.

Option 2: The outer core is a layer found beneath the mantle and it is composed of liquid iron and nickel.

Option 4: The inner core is a solid layer located at the center of the Earth, beneath the outer core.

Therefore, the correct answer is option 3, the crust, as it is the thinnest layer compared to the other options.