In this sentence, according to the context, the verb “to find out” means to discover something to the point of knowing it, in simple words: To know something.
So, in spanish, the verb “to know” means “saber” or “conocer”, in this context is the first one:
Saber
This sentence is in simple past tense. The past tense of “saber” with the subject pronoun “Yo” (I) is: “supe”
Then, the correct translation option is:
Supe el secreto ayer