Clarification about Market Structure Oligopoly or Monopoly As per the definition i found in the internet, 


Oligopoly, in which a market is run by a small number of firms that together control the majority of the market share. 
Monopoly, where there is only one provider of a product or service.


Consider this situation in a country.

There are 4 main bus operators in this country. A,B,C and D and some individual bus owners.

The bus fare is controlled by a national body.
Some particular bus owners  who has AC installed in the bus, charges a 0.2$ extra per route.

In the above scenario in what classification would you put the transport sector of this country. Is it a monopoly as the price is regulated by a single body? or is it an oligopoly as the market share is dominated by these four main company. 

Further more for my upcoming exam if I get a question on Market structure of this country transport system how can I elaborate?
This market is an oligopoly that is subject to government regulation. 

It cannot be a monopoly because there is more than one firm. The presence of the regulating government body is a "red herring", it distracts from the main point- there are multiple firms. 

It does not appear to be competitive because:


Four is subjectively few firms
Implicitly, regulation would be entirely pointless in a competitive market.
One firm has managed to differentiate themselves by adding air conditioners. Firms in perfect competition cannot be differentiated from one another.