The correct answer is A. few.
The word “few” means a small number of people or things. In this case, the speaker is saying that they have never been to Japan and they know a small number of people there.
The word “a few” also means a small number of people or things, but it is slightly more informal than “few.” In this case, the speaker could have used “a few” if they were talking to a friend or family member.
The word “many” means a large number of people or things. In this case, the speaker is saying that they know a large number of people in Japan. This is not likely, since they have never been to Japan.
The word “most of” means almost all of the people or things in a group. In this case, the speaker is saying that they know almost all of the people in Japan. This is also not likely, since they have never been to Japan.