Which best describes why kepler’s observation of planetary motion is a law instead of a theory?

Theory is a description or explanation of a fact or occurrence obtained through experimentation, testing, observation whereas law is a statement which gives/tells relationship between entities under certain conditions. 
Kepler’s observation of planetary motion is a law not a theory because we assume certain things before deriving a mathematical expression. If you remember, the time period of a satellite moving around a planet is derived by assuming its orbit to be circular. We have "assumed the orbit to be circular" hence it is a law. Same with Einstein's relativity. Relativity is not a theory! You might have seen books titled "Einstein's Theory of Relativity" but it is "Einstein's law of Relativity" because he assumed certain axioms before putting forth his work. Hope this helped you.

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