Final answer:
The industry with a horizontal long run supply curve and constant average costs as production expands is the perfectly competitive industry or constant-cost industry. Firms in this industry will make zero economic profits in the long run, and the market price is determined by where demand meets the firms' minimum average cost.
Step-by-step explanation:
The industry in which a typical firm's average costs do not change as the industry expands production, and which will have a horizontal long run supply curve, is a perfectly competitive industry, known as a constant-cost industry. In the long run, a perfectly competitive market experiences a process of entry and exit that drives the price to the zero-profit level, also called the competitive equilibrium, where price equals both marginal cost and average cost. This ensures that in the long run, firms in a perfectly competitive market will only make normal profits, which are zero economic profits. The long-run supply curve in such an industry is horizontal, indicating that the market supply does not affect the price; rather, it's determined by the market demand intersecting with the individual firms' minimum average cost.
Conversely, in increasing-cost industries, costs increase with industry expansion, leading to an upward-sloping supply curve. In decreasing-cost industries, costs decrease as the industry expands, leading to a downward-sloping supply curve. The other market structures, such as monopolistic, monopolistically competitive, and oligopolistic, do not fit the criterion for having a horizontal long run supply curve because they either have some degree of market power or product differentiation, preventing them from reaching the constant-cost scenario.