The correct option is A 20 millilitre
Teaspoon is used for measurement of small capacity liquids.
20 millilitre water is approximately equal to amount of water in a small sized bowl.
20 Litre water is approximately equal to amount of water in a bucket.
20 Kilolitre water is approximately equal to amount of water in 1000 such bucket.
Hence, 20 ml is the closest to 1 teaspoon.