Answered

Under feudalism, which group was responsible for ruling different regions of a country? peasants knights lords king

Answer :

Lords because the king ruled the whole country. Pheasants did farm work and had no rule and knights fought for the king and got small amounts of land.
Answer
Under feudalism, the lords, was responsible for ruling different regions of a country. 

Explanation; 
Feudalism is a type of society and the political system originating from Western and central Europe and dominant there during the Middle Ages.
The lords had certain roles for the feudal system to work these includes;
Serving the royal council, providing the King with Knights to defeat any kind of war, providing food and shelter and other necessities to the king, and they had to pay taxes and fees to the king if he needed them.