Choice "B" is correct. In microeconomic analysis, in the long run all supply side inputs are variable. In accounting terms, this means that in the long run all costs are variable. (e.g., the fixed cost of depreciation of a factory building becomes a variable cost when a second factory building is added.)
Choices "a" and "d" are incorrect. Output and price are ultimately determined by market factors (supply and demand) and the power a firm has in the market.
Choice "c" is incorrect. In competitive markets (including monopolistic competition), firms rather easily enter and exit the market.
