a) Deep sea fishes die when suddenly brought to surface. The pressure at the bottom of the sea is more, but the pressure at the surface is less. So due to low pressure the fish dies.
b) Scuba diver’s life is under threat when they suddenly come to the surface quickly. As a diver goes deeper underwater, that pressure begins to increase. With the increase in pressure volume decreases, nitrogen gas begins absorbing into the diver's blood. When the diver begins his ascent, these gas molecules begin to expand back to their normal volume. With a slow ascent, or through the use of a depressurization chamber, those gasses can work their way back out of the bloodstream as they return to their normal volume.
c) Balloons pop when squeezed. When a balloon is squeezed, the pressure increases. So it pops.
d) We feel uneasy pain in the ears during take off and landing of aeroplanes. When the aeroplane takes off or lands, there is imbalance in pressure inside and outside of eardrum.