The idea that the earth's crust is made of thick plates which slide around on top of the mantle

That would be the theory of plate tectonics 

The dictionary meaning of the phrase is: a theory explaining the structure of the earth's crust and many associated phenomena as resulting from the interaction of rigid lithospheric plates that move slowly over the underlying mantle.

Hope this helps!

Each side has to be less then the sum of the other two sides.a, b, c, and e work.

Bis the answer i believe