Yes I am clear it won't be on the exam, but considering the question contains less competitive markets, the only way to explain it is by oligopoly market structures so on, it is hard not to refer to elements from those markets
That is true, but as it isn't on the study design I'm confused on why you want to answer it.
Lets say you pick a monopoly.
Its effect on price would be that it would set prices that create "super-normal profits", but for consumers is bad new bears because some of them will have to drop out of the market as they cannot afford these higher prices.
For the efficency types, you can mention how there will be a lower productive efficiency because they are the only producer of this product, which means they have little to no pressure to improve the productivity of producing this product despite the benefits it could have for the market.
Lower allocation of resources aka allocative efficiency: this is due to the factor of usually under-allocating resources to producing their product, so they can raise prices on them for consumers, who then pay that elevated price or some consumers won't purchase the product despite needing it due to the inflated price set by the monoplist.