Answer: A

The sentence is correct as written. Choice (B) deletes the word "thereby," which is important as it signals the causal relationship between marrying Poland's ruler and becoming King of Poland himself. (C) maintains that relationship, but adds a new one, suggesting that he converted to Christianity as a result of the marriage. This is inaccurate. Choices (D) and (E) change "was known as" to "was," which changes the meaning. Choice (A) is correct.