We have been in Sector: 8, Gandhinagar . . . . . . 1997.

Prepositions of time indicate the duration or point in time of an action or state. "For" indicates a duration of time. "From" indicates a starting point in time. "About" indicates an approximation of time. "Since" indicates a point in time from which an action or state has continued to the present. The correct answer is D: since. The sentence describes a continuous state (being in Sector 8, Gandhinagar) that began in 1997 and continues to the present time when the sentence was spoken or written. "Since 1997" accurately reflects this continuous state. Option A, "for," is incorrect because "for" requires a duration, not a starting point. We would need to specify a duration, e.g., "We have been in Sector 8, Gandhinagar for 25 years." Option B, "from," is incorrect because "from" only indicates a starting point. It doesn't convey the continuous nature of the state. It would imply they moved there in 1997 but might have left since then. Option C, "about," is incorrect because it suggests an approximation. The sentence indicates a precise year (1997), making "about" inappropriate.