Answer:
The correct answer is: a single firm producing a product for which there are no close substitutes.
Explanation:
A pure monopoly is a market structure where there is only a single producer or firm. The firm produces a product which has no close substitutes. The firm is a price maker and faces a downward-sloping demand curve.
When a standardized product is made by a large number of firms it is a perfectly competitive market.
When a large number of firms make differentiated products. It is a monopolistic market.