The correct matches would be as follows:
1. Reading with the most water in the air.
c. Humidity of 90%
2. Reading with 80% water in the air.
b. Humidity of 80%
3. Reading that shows the air could hold 80% more water.
a. Humidity of 20%
4. Reading with the lowest amount of water in the air.
d. Humidity of 10%
Hope this answers the question. Have a nice day.