The correct answer is since.
The word “since” is used to indicate a point in time in the past from which something continues to happen or be the case. In this case, the speaker is indicating that the person has been living in Switzerland for two years, and that this has been happening continuously since a point in time two years ago.
The other options are not correct because they do not indicate a continuous action. The word “from” is used to indicate a starting point, but it does not indicate that the action continues after that point. The word “for” is used to indicate a period of time, but it does not indicate that the action continues after that period of time. The word “until” is used to indicate a point in time in the future at which something will stop happening, but it does not indicate that the action has been happening continuously up to that point.