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 they have been in Sector 8, Gandhinagar since 1997. This means that they have been living there continuously since that year.
The other options are not as appropriate in this context. The word “for” is used to indicate a period of time during which something happens or is the case. In this case, the speaker is not indicating a specific period of time, but rather a continuous state of being. The word “from” is used to indicate the starting point of something. In this case, the speaker is not indicating the starting point of their stay in Sector 8, Gandhinagar, but rather the fact that they have been there continuously since 1997. The word “about” is used to indicate an approximate amount or number. In this case, the speaker is not indicating an approximate amount of time, but rather a continuous state of being.