Into which layer of the earth do plate boundaries extend? outer core lithosphere lower mantle asthenosphere

The answer is lithosphere,which is the outermost shell of the Earth, where we're living on and composed of mainly of rocks, and will extend due to the convection movement of magma.

Let's look into the other answers:

Outer core: It is the second deepest layer in Earth, it is not visible for us. It is consisted of molten rock materials with high density. As the temperature is very high (4400-6100°c) the mantle cannot be cooled down to form plate material. Therefore thos answer is incorrect.

Lower mantle: Lower mantle is under the lithosphere and is solid however can flow freely. It is not directly involved in the extension of plate boundaries and is therefore false.

Asthenosphere: It is a layer of molten magma which is indirectly responsible for the plate movement by causing the convection current of magma plate . However this is not exactly where the plate boudaries take place and therefore is not the answer.

How lithosphere is involved in plate boundaries extention: Given the rising convection current of magma, the crust move apart and leave cracks whoch are then filled by magma. As magma cools down,new plates material is cooled down to form new crust. Under ridge push and slab pull, the plate materials is extended.

Hope it helps!

The correct answer should be brand c.