Minimum efficient scale is the output level at which a firm has exploited the main economies of scale and achieved the lowest long-run average cost of production. Up to this point, expanding output tends to reduce unit costs as fixed costs are spread and operational efficiencies improve. Once the firm reaches minimum efficient scale, further expansion does not necessarily reduce long-run average costs and may eventually introduce diseconomies of scale, such as coordination issues, management complexity, and rising inefficiencies. In the theory of the firm, the strategic implication is that the firm has reached a scale where cost advantages from growing larger are no longer the primary driver of competitiveness. Therefore, the firm should not expand output purely to chase lower unit costs. It would typically focus on maintaining efficient operations, protecting market position, and only increasing output if it can still do so profitably without pushing costs higher. That makes halting output expansion, in the context of cost-minimisation theory, the best answer among the options provided.