When Fortin barometer is taken to a high mountain. So, the mercury falls in the tube, because

Option C: When a Fortin barometer is taken to a high mountain, the mercury falls in the tube because atmospheric pressure becomes lower there. Atmospheric pressure decreases with altitude due to the reduced column of air above. This lower pressure causes the mercury column in the barometer tube to drop, indicating the change in pressure. Unlike options A, B, and D, which don’t directly relate to the mechanism of barometric pressure changes, option C aligns with the fundamental principle of barometric measurement. As the altitude increases, the weight of the air column above decreases, resulting in lower atmospheric pressure. This pressure difference causes the mercury in the barometer tube to fall until it reaches equilibrium with the surrounding atmospheric pressure. Therefore, the decrease in atmospheric pressure at higher altitudes is the primary reason for the mercury falling in the Fortin barometer tube. Understanding this principle is crucial for interpreting barometric readings accurately, especially when measuring atmospheric pressure variations in different geographical locations and altitudes.