The syntax for this question seems rather odd, and one wonders if there might not be some error. There are no other examples in the Book of Mormon of this kind of construction involving the verb thank (which otherwise occurs as a verb 16 times in the text, as in Mosiah 2:19: “O how had you ought to thank your heavenly King”). Of equal frequency is the expression “to give thanks”, which occurs 17 times in the text. One could, for instance, revise the text here in 2 Nephi 29:4 so that it reads “and what thanks do they give the Jews for the Bible” or perhaps the archaic “and what thanks give they the Jews for the Bible”. It seems doubtful, however, that either of these revisions represent the actual original text for 2 Nephi 29:4, although the second would involve the least amount of change (accidentally omitting the verb give and dropping the s of the word thanks).
The King James Bible, however, does have three examples of “what thank” (and all in the same passage), but in these examples the word thank is a noun rather than a verb:
There is, however, one example in the King James Bible where the verb thank is used somewhat similarly to how it is used in 2 Nephi 29:4:
In this case we have the word order thank , just as in 2 Nephi 29:4 (“thank they the Jews”). But one important difference is that there is no direct object in the biblical passage that corresponds to the fronted what of 2 Nephi 29:4 (“what thank they the Jews”); instead, there is the prepositional phrase “for this cause”. A perfectly equivalent construction would be if 1 Thessalonians 2:13 read “this cause also thank we God without ceasing” (in other words, if the preposition for were lacking). In any event, the Book of Mormon construction does have a for, but it occurs later in the clause (“and what thank they the Jews for the Bible”).
Obviously, the construction in 2 Nephi 29:4 seems very unusual. It seems that we have a case of the verb thank taking two different objects, with what acting as the direct object and the Jews as the indirect object. The Oxford English Dictionary lists such a construction under definition 4a of the verb thank, describing the use of “dative of person (indirect object) and accusative thing (direct object)” as obsolete. The last instance cited there in the OED is from late Middle English (and in Scottish dialect):
The Taill of Rauf Coilyear [The Tale of Ralph the Collier] (about 1475)
Mair the King spak nocht, Bot thankit thame thair deid. ‘more the king spake naught but thanked them their deed’
Given such examples in earlier English, it is perhaps safest to assume that the strange use in 2 Nephi 29:4 of “what thank they the Jews” is intended and not the result of an error in transmission. We further note that no edition or manuscript has shown any attempt to revise the text here so that it might read more acceptably.
Summary: Retain the unusual usage of “what thank they the Jews” in 2 Nephi 29:4; this construction appears to be very archaic but is nonetheless understandable.