There are molecules of air at high elevations, so air pressure is less.
a. fewer
c. more
b. the same number of
d. no

So we want to know how does the number of moleculs and with it the pressure change with altitude. So the air is the most dense, or the number of air molecules is the biggest on the lowest altitude and that is the ground. As we go higher, the number of molecules per cubic metre is falling and there are FEWER molecules the higher we go. So since pressure is number of collisions per some surface area, the fewer the molecules, the fewer the collisions and so the smaller the pressure gets. The correct answer is a.

Youre answer is A ; fewer

Wavelength divides in half and speed stays the same.