Question

What is the theory of plate tectonics based upon?

Answer 1

The Mantle!

Explanation:

sorry, geology jokes are few and have to be grabbed where possible.
to be serious, the most convincing piece of evidence are the magnetic bands with the rock on the Atlantic see floor.

Lots of people had argued that plate tectonics were responsible for mountain ranges etc. but the counter argument of energy required was often used to shout that argument down. Whereas the magnetic bands on the Atlantic sea floor presents a very strong basis for this theory, that is difficult to counter.

if you want an explanation of these magnetic bandings i suggest you try:
US geological survey, a little dry but very trustworthy
[teaching resource, better descriptions]
(https://www.ck12.org/earth-science/Magnetic-Evidence-for-Seafloor-Spreading/lesson/Magnetic-Evidence-for-Seafloor-Spreading-HS-ES/)