Option 1: 5 -> At equilibrium price P = 15, market demand is Q_d = 180 - 2(15) = 150 units. Each firm supplies Q_s1 = 15 + 15 = 30 units. Since total market supply must equal market demand in equilibrium, the number of firms = 150 ÷ 30 = 5 firms. This ensures market clearing where aggregate supply from all identical firms exactly matches market demand. -> correct