The answer is: They are the Jews and Christians as stated by the scholars of Tafsir (exegesis of the meanings of the Qur'an) and others. As for the Magi, they are not of the people of the Book but the same rule of Jizyah (poll tax required from nonMuslims living in an Islamic state) applies to them. The Prophet (peace be upon him) did so. According to the Four Imams (Abu Hanifah, Malik, AlShafi`, and Ahmad) and others, Muslims are forbidden to marry Magian women or eat the flesh of the animals slaughtered by the Magi. This point is unanimously agreed upon among the scholars. The view that these two matters are lawful is odd and among scholars who stated this is Abu Muhammad ibn Qidamah (may Allah have mercy on him) who says in his book AlMughny,
( The People of the Book that are included in this ruling are the people of Tawrah (Torah) and Injil (Gospel) as Allah
(Exalted be He) states, Lest you (pagan Arabs) should say: "The Book was sent down only to two sects before us (the Jews and the Christians) The people of Tawrah are the Jews, Samaritans, and the people of Injil are the Christians and their followers among the Europeans, Armenians, and others ... the Magi have no revealed Book, so Muslims are forbidden to marry Magian women or eat the flesh of the animals slaughtered by the Magi. This is stated by Ahmad and agreed upon by the majority of scholars except Abu Thawr who considered this lawful because of the Prophet's (peace be upon him) statement, Treat them like you treat the people of the Book. This is also because of the report which states that Hudhayfah married a Magian woman and for the fact that the Magi pay Jizayah like the Jews and Christians.
Allah (Exalted be He) states, And do not marry AlMushrikât (idolatresses, etc.) and Likewise hold not the disbelieving women as wives Muslim males are permitted to marry women of the People of the Book and those who are not of the people of the Book are forbidden. There is no evidence that the Magi have a book. Moreover, Ahmad was asked: Was it authentically reported from `Ali that a Book was revealed to the Magi? He replied that this is absolutely false. Even if they have a Book, it is evident that rules applied to the People of the Book are not applicable to those other than them. The Prophet's statement, Treat them like you treat the people of the Book... proves that they have no Book. The Prophet (peace be upon him) wanted to prevent bloodshed and let them pay Jizayah. Since their having a Divine Book is a doubtful matter, Muslims protect their lives. Thus, it is forbidden to marry a woman from them or to eat from the animals slaughtered by them. Consequently, we must favor the evidence over the opposing doubtful matter. It is not authentically established that Hudhayfah married a Magian woman. Ahmad classified the report stating that Hudhayfah married a Magian woman as weak and said that Abu Wa'il, who is more reliable than the narrator who gave the above report, mentioned that Hudhayfah married a Jewish woman. Ibn Sirin said that Hudhayfah's wife was Christian. Owing to these contradicting narrations, none of them can be authentic unless it is confirmed. Even if it is authentically established that Hudhayfah married to a Magian woman, it is impermissible to use this fact as a proof and go against the Book and the view of the other scholars.
With regard to the issue of paying Jizayah, it is based on the prevailing ruling of protecting their lives. Therefore, prohibiting marriage to their women or eating their slaughtered animals must prevail. End quote of AlMughni's author (May Allah have mercy on him). Finally, Allah knows best.