(a) The height of the blood column in the human body is more for the feet as compared to that for the brain that is why the blood exerts more pressure at the feet than at the brain.
(b) We know that the density of air is maximum near the surface of earth and decreases rapidly with height and at a height of about 6 km, it decreases to nearly half of its value at the sea level. Beyond 6 km height, the density of air decreases very slowly with height. Due to this reason, the atmospheric pressure at a height of about 6 km decreases to nearly half of its value at sea level.
(c) Since due to the applied force on the liquid, according to Pascal’s law, the pressure is transmitted equally in all directions inside the liquid. Thus there is no fixed direction for the pressure due to liquid. Hence hydrostatic pressure is a scalar quantity.