Option 1: "It remains constant" is not the correct answer. Atmospheric pressure does not remain constant with an increase in altitude. As we go higher in the atmosphere, the weight of the air above decreases, thus decreasing the atmospheric pressure. So, option 1 is incorrect.

Option 2: "It decreases" is the correct answer. As we move higher in the atmosphere, there is less air above us, leading to a decrease in the weight and density of the air molecules. As a result, the atmospheric pressure decreases with increasing altitude.

Option 3: "It increases" is not the correct answer. In reality, the atmospheric pressure decreases with an increase in altitude, so option 3 is incorrect.

Option 4: "It constantly fluctuates" is not the correct answer. Atmospheric pressure may naturally fluctuate due to weather conditions, but the general trend with increasing altitude is a decrease in pressure. So, option 4 is incorrect.

In conclusion, the correct answer is option 2 - "It decreases."