Of these choices, it is more correct to say that Wal-Mart is in an oligopoly.
The clearest reason to say this has to do with the definition of a monopoly. In a monopoly, there is only one seller for an entire market. This is not true of Wal-Mart's market. Wal-Mart must compete with other similar stores such as K-Mart and Target and ShopKo. It must also compete with other stores that carry particular items that Wal-Mart carries. For example, Wal-Mart must compete with stores like Best Buy and Staples in selling electronics. It must compete with internet retailers like Amazon in a number of areas. Because there is so much competition, Wal-Mart cannot be considered a monopoly.