Correct option is A
Option (a) is the correct meaning of the given idiom.
In this context, "rule the roost" means that the country's nationalists are in control or in a position of authority, and they are the ones making decisions and assigning jobs.
It signifies being in a dominant or leading position.
Example: In the company, the experienced project manager rules the roost when it comes to decision-making and resource allocation, as his expertise is highly valued by the team.