ANSWER :
C – The translators got carried away.
The phrase “became Jews” is one word in Hebrew: mitehayadim מִֽתְיַהֲדִ֔ים, which is the verb form of Jew; it does not mean they literally became Jews, but rather that these Persians became true worshipers of God.
A quick look at David’s writing will help us. When the Psalmist says, “Praise the Lord,” the word is yadah, from which the word Jew, yehudi, derives. In Esther, the word mitehayadim indicates that these Persians became true worshipers of God.
According to the Soncino Press Commentary on Esther by A. Cohen, we read, “Since nowhere else, either in the Bible or in rabbinic Hebrew, is this verb used to denote proselytization, it may be argued with some reason that a better translation is –took the part of the Jews.”
One can convert to rabbinical Judaism, but one cannot become a Jew according to the biblical definition: In the New Testament, those who joined the community of Israel were called proselytes, not Jews.