Ink comes out from a fountain pen in an airplane because air pressure decreases with an increase in altitude ; option [A]. As the aircraft ascends to higher altitudes, the atmospheric pressure surrounding the pen decreases. However, the pressure inside the pen remains relatively constant due to the sealed environment. This pressure disparity causes the air inside the pen to expand, creating a higher pressure inside than outside. Consequently, the increased pressure inside the pen forces ink out of the nib, leading to ink leakage. This phenomenon occurs because gases, like the air inside the pen, tend to expand when subjected to lower pressure environments. Therefore, the decreasing atmospheric pressure at higher altitudes allows the air inside the pen to expand, pushing the ink outward. As a result, passengers may experience ink leaks from fountain pens during flights, particularly during takeoff and ascent when the altitude rapidly increases, causing a significant drop in atmospheric pressure. Thus, the correct answer is [A] air pressure decreases with increase in altitude.