The equation \[E = m{c^2}\] was theoretical. It received experimental proof from.
A. Nuclear reactions carried out in the laboratory
B. Measurement of molecular mass of a compound formed by chemical combination
C. Measurement of molecular mass of products of decomposition
D. The phenomenon of radioactivity.

Hint: Einstein's Mass-energy Relation Mass-energy equivalence states that every object possesses certain energy even in a stationary position. Though from Newtonian mechanics we have learned a stationary body does not have any kinetic energy but only have potential energy and probable chemical and thermal energy.The relation of mass energy equivalence was not proved initially when Einstein wrote his paper. But, later on it was proved.

Complete answer:
If we look at the history of physics, especially the era of Einstein, Mass-energy equivalence arose from special relativity as a paradox described by the French polymath Henri Poincaré.Then Einstein was the first who proposed the equivalence of mass and energy as a general principle and a consequence of the symmetries of space and time. Factually, the equation \[E = m{c^2}\] was found theoretically and it was not proved initially. Later the theorem received the proof from the phenomenon of radioactivity.

Hence, option D is correct.

Note: Few usage of Einstein’s Mass-energy equivalence theory are:
-Einstein’s theory was used to understand nuclear fission and fusion reactions also it was proved experimentally from there.
-It was also used to find the binding energy in an atomic nucleus
-It was used to understand the effect of gravity on distant celestial bodies and so on.