Single best answer: A  

Concise reasoning: Setting mandibular posterior teeth too far lingually steals tongue space; this leads to tongue interference and articulation problems, and shifts occlusal loads off the crest of the ridge, increasing localized pressure on the mucosa. Gagging, however, is chiefly provoked by over-extension or thickening of the maxillary denture posteriorly, not by the lingual position of lower teeth, so it is the least likely consequence. [Jiang, 2001, PMID 14994031; Żmudzki, 2019, PMID 29285830; Rehmann, 2017, PMID 28744904]