The original text frequently has the prepositional a before the present participle in a verb phrase. Historically, the a derives from the preposition on (with the meaning ‘in’) and precedes a gerundive nominal, so that “they were a scoffing at them” meant something like ‘they were in [the process of] scoffing at them’. Such uses are now considered either archaic or dialectal. Examples can be found in the King James Bible, such as Peter’s declaration “I go a fishing” (John 21:3). Such uses of a have all been removed from the Book of Mormon text, usually as a result of explicit editing. In this example from 1 Nephi 8:28, the a was deleted by the 1830 compositor. Most of the time, the original text does not use the prepositional a, and one could view its occasional appearance as a dialectal overlay. Nonetheless, the critical text will restore these a’s whenever there is support from the earliest textual sources. For a complete listing and analysis, see prepositional a in volume 3.
Summary: Restore the prepositional a in 1 Nephi 8:28 (“those that were a scoffing at them”) since the a is found in the manuscripts.