Step 1: Read the question.
The blank asks what humidity is the amount of in the air. We must pick the correct substance.
Step 2: Meaning of humidity.
Humidity is a weather word. It tells us how much invisible water in gas form is present in the air around us.
Step 3: What water vapour is.
When water evaporates from rivers, seas, and plants, it turns into water vapour, which is the gas form of water. This gas mixes into the air.
Step 4: Connect to feeling sticky.
On a damp, sticky day the air holds a lot of water vapour, so we say humidity is high. On a dry day there is little water vapour, so humidity is low.
Step 5: Rule out the others.
Oxygen and nitrogen are always in the air in fixed large amounts and are not what humidity measures. Dust is solid particles, not the meaning of humidity.
Step 6: Conclude.
Humidity is the amount of water vapour in the air.
\[ \boxed{\text{Water vapour}} \]