ABA contains

The correct option is "one asymmetric carbon atom".

To understand why this is correct, let us analyze the chemical structure of ABA (Abscisic Acid). Abscisic acid is a naturally occurring plant hormone (phytohormone) involved in various developmental processes, including seed dormancy and responses to environmental stresses like drought.

The chemical formula of abscisic acid is $C_{15}{H}_{20}{O}_4$.
Its IUPAC name is (2Z,4E)-5-[(1S)-1-hydroxy-2,6,6-trimethyl-4-oxocyclohex-2-en-1-yl]-3-methylpenta-2,4-dienoic acid.

Let us look at the structure of the cyclohexenone ring system in abscisic acid. The ring contains a carbon atom at position 1' (the carbon bearing the tertiary hydroxyl group, -OH, and the side chain).
Let's check the four groups attached to this carbon atom (C-1'):
1. A hydroxyl group (-OH)
2. A conjugated carbon side chain (-CH=CH-C(CH₃)=CH-COOH)
3. The carbon C-2' of the ring, which is part of a double bond system (-CH=C(CH₃)-)
4. The carbon C-6' of the ring, which is a saturated carbon substituted with two methyl groups (-C(CH₃)₂-)

An asymmetric carbon atom (or chiral center) is a carbon atom that is bonded to four different chemical groups.
Since all four groups attached to C-1' are chemically distinct, this carbon atom (C-1') is a chiral center (an asymmetric carbon atom).

No other carbon atom in the abscisic acid molecule is bonded to four different groups:
- The methyl carbons are bonded to three hydrogen atoms.
- The C-6' ring carbon is bonded to two identical methyl groups, making it symmetric.
- The sp² hybridized (double-bonded or carbonyl) carbons in the ring and side chain are bonded to only three groups, so they cannot be asymmetric.

Therefore, abscisic acid (ABA) contains exactly one asymmetric carbon atom.