The correct response is:
Christianity and Judaism are monotheistic religions.
This accurately reflects a key similarity between the two faiths, both of which believe in one God. The other options contain inaccuracies; for example, the Tanakh is primarily the holy text of Judaism, while Christians refer to the Bible, which includes the Old Testament (similar to the Tanakh) and the New Testament. The Sabbath is a significant day of rest in Judaism but is not universally observed in Christianity. Lastly, Christians and Jews do not believe in more than one god.