Which best describes the relationship between humidity and air pressure?
humid air has higher pressure because of the heaviness of water.
humid air is lighter, so it has lower pressure.
air pressure is lower below sea level because of the humidity.
humidity and air pressure are not relate

Humid air is lighter, so it has lower pressure.

The reason is the molecules of water are H2O, whose molar mass is 18 g/mol.

These molecules displaces molecules of N2 and O2, whose molar masses are:

N2: 2*14g/mol = 28 g/mol, and
O2: 2*16g/mol = 32 g/mol.

Then molecules of 28g/mol and 32 g/mol are being replaced with molecules of 18g/mol, leading to a lower weight of the same volume of air, which results in lower pressure.