The correct answer is option 1: 1200-1300 gm. The human adult brain weighs approximately this much. This weight range is considered normal for the average adult brain. Option 2, 1600-2000 gm, is incorrect as this is a higher weight than what an adult human brain typically weighs. Option 3, 500-800 gm, is also incorrect as this weight range is considerably lower than the average weight of an adult human brain. Option 4, 100-200 gm, is the most inaccurate as this weight range is far too low for a human adult brain. It is important to note that individual brain weights can vary slightly, but the provided answer of 1200-1300 gm is the most accurate range for the average adult human brain.