Which of the following could lead to static discharge?
water molecules sticking together at a surface
the force of gravity pulling an object to Earth
a vibration compressing molecules of air
the build up of electrons on the atoms of an object

Static charge is caused by the build up of electrons on the atoms of an object.

Explanation:

You can experience this yourself with a balloon! Rub it against your head in order to disassociate electrons from it, causing it to be charged - you can now put it on a wall and notice how it 'sticks' to it. You can also cause this effect by jumping on a trampoline, causing static charge to be built up - which is sometimes why you feel a little 'zap' when you touch the sides of it.

c, liquids evaporate, creating a gas