The correct answer is A. inaccessible.
Inaccessible means not able to be reached or entered. This is the best word to describe the regions of our country that are still difficult or impossible for the average person to reach.
Impossible means not able to be done or achieved. This word is too strong to describe the regions of our country, as it is possible for people to reach them, but it may be difficult or require special skills or equipment.
Impervious means not allowing anything to pass through. This word is not a good fit for the regions of our country, as they are not completely closed off from the outside world.
Impenetrable means impossible to pass through or penetrate. This word is also not a good fit for the regions of our country, as they are not completely impossible to reach.