Answer: Monopoly
Explanation: In simple words, monopoly refers to a market structure in which there is only one participant in the market that is offering that particular product or service.
Generally, such kind of market structure arises due to patent rights or some innovation etc. Although there is only one participant in such structures, it can not over price it commodities as the law of demand applies to such structure also.
Hence from the above we can conclude that a monopoly is occurring in such a structure.