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John Calvin himself wrote in his commentary of the Bible, pertaining to John chapter 6, verse 40 (emphasis mine):
He uses the words, see and believe, in contrast with what he had formerly said; for he had reproached the Jews with not believing, even though they saw, (verse 36.) But now, speaking of the sons of God, with the feeling which they have of the power of God in Christ, he joins the obedience of faith. Moreover, these words show that faith proceeds from the knowledge of Christ; not that it desires anything beyond the simple word of God, but because, if we trust in Christ, we must perceive what he is, and what he brings to us.
Calvin is saying here that Jesus’s words should be seen in the context of the overall passage. He is speaking of the Jews at the present time, in contrast to those who had seen yet not believed. He isn’t making such a broad statement about all human history like A or B. In short:
C. The Jews have seen. Those who believe as a result of their seeing will be saved; those who have seen and don’t believe won’t be saved.
This question and its answers may lend some assistance in understanding how people prior to Jesus were saved, but the above commentary sums up the Reformed interpretation of John 6:40.