In economics, a monopoly is a market structure that is characterised by which of the following features?

Introduction / Context:
This question examines the basic definition of a monopoly in microeconomics. Market structures such as perfect competition, monopolistic competition, oligopoly and monopoly are classified based on the number of sellers and buyers, the nature of the product and the ease of entry and exit. A monopoly represents one extreme, where a single firm has significant control over the supply of a product or service. Understanding this definition is necessary for analysing pricing power, welfare effects and the need for regulation.

Given Data / Assumptions:
- The term being defined is "monopoly".
- The options describe markets with a single seller, a single buyer, a single seller and buyer pair and a seller with no buyer.
- We assume standard textbook definitions of monopoly and monopsony.

Concept / Approach:
A monopoly is a market structure in which there is a single seller of a product or service that has no close substitutes, and this seller faces many buyers. The monopolist can influence the price because consumers have few or no alternative suppliers. In contrast, a monopsony occurs when there is a single buyer and many sellers. A one to one relationship between a single seller and a single buyer is not the standard definition of monopoly but rather a bilateral monopoly. A market with a seller but no buyer is simply not a functioning market. Therefore, the definition that mentions a single seller dominating the market fits the concept of monopoly.

Step-by-Step Solution:
Step 1: Recall that the key feature of a monopoly is a single seller with market power over a particular product or service. Step 2: Identify the option that explicitly states "single seller that dominates the market". Step 3: Recognise that "single buyer" refers to a monopsony, not a monopoly, so option B is not correct. Step 4: Note that a single seller and single buyer (option C) describe a bilateral monopoly, a special case not equal to the general definition of monopoly. Step 5: Understand that a seller with no buyer (option D) does not represent a meaningful market structure at all. Step 6: Select option A as the correct description of a monopoly.

Verification / Alternative check:
To verify, think of classic examples often used in textbooks: a public utility company in a small town, a patented drug held by a single pharmaceutical firm or a railway operator with exclusive rights on a route. In each case, there is one dominant seller serving many buyers. The buyers do not have close substitutes available, which gives the seller market power. None of these examples involve a single buyer or a one to one pairing, reinforcing that the essential characteristic is a single seller dominating supply.

Why Other Options Are Wrong:
Option B describes a monopsony, where one buyer purchases all the output from many sellers. This is a different concept used in labour and input markets.
Option C describes a bilateral monopoly, which is far more specific and involves both a monopolist seller and a monopsonist buyer, not the general case of monopoly.
Option D suggests a situation where there is a seller but no buyer, which fails to define a working market and is not an accepted market structure in standard theory.

Common Pitfalls:
Some learners confuse monopoly with monopsony because both words describe market power but on opposite sides of the market. Others may think any situation with one dominant participant on either side qualifies as monopoly. To avoid such mistakes, always remember that monopoly refers to one seller, while monopsony refers to one buyer. The prefix "mono" means "one", and the root "poly" in this context comes from "polein", meaning "to sell". This etymology supports the idea that monopoly is about a single seller.

Final Answer:
A monopoly is a market structure that has a single seller that dominates the market for a particular product or service.