La teoría de la relatividad fue descubierta por Einstein, is the right answer.
the first choice is wrong because "es" means is; it is not passive.
the second choice because "será" means "will be"; it is not passive.
The third choice is passive tense, "fue". However, "descubrida" is not proper.
The only right choice is the right choice; passive tense and proper use of the word.
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