One may wonder here in Mosiah 21:26 if the last occurrence of the relative pronoun which refers to the land that was destroyed or to the people that inhabited that land. Usually the Book of Mormon refers to people being destroyed. There are two instances in quotations from the King James Bible where the Book of Mormon text refers to the land being destroyed:
There is one nonbiblical passage which parallels the language of Mosiah 21:26:
The text in Alma 22:30 clearly refers to “the land which had been peopled and had been destroyed”, but the following “of whose bones we have spoken” seems to refer to the bones of these people (the Jaredites) rather than the bones of the land (although that interpretation is not impossible). Similarly, there is a reference to the bones of the Jaredites in Mosiah 21:27 (which immediately follows the verse under consideration, Mosiah 21:26): “and they brought a record with them / even a record of the people whose bones they had found”. It seems that in both Mosiah 21:26–27 and Alma 22:30, when the text refers to the land which was destroyed, it really means that the people of the land were destroyed, thus the shift in both cases to mentioning the bones of the people who were destroyed.
The critical text, of course, does not need to determine whether the which in Mosiah 21:26 refers to the land or the people as having been destroyed, although this distinction could become an issue in translating the text into another language or in determining whether which should be grammatically emended to who in the standard text. Given the specific usage in Alma 22:30, it is most reasonable to assume that the antecedent for the last which in Mosiah 21:26 is indeed the land. For a complete discussion of the editing of the relative pronoun which, see which in volume 3.
Summary: Maintain the relative pronoun which whenever it is supported by the earliest textual sources; here in Mosiah 21:26, as in Alma 22:30, the antecedent for which is technically the land.