Intramolecular hydrogen bonding is found in:A. salicylaldehydeB. waterC. acetaldehydeD. phenol

Intramolecular hydrogen bonding refers to the formation of hydrogen bonds within a single molecule. Among the options you provided, the compound that exhibits intramolecular hydrogen bonding is salicylaldehyde (option A).

Salicylaldehyde (also known as 2-hydroxybenzaldehyde) contains a hydroxyl group (-OH) and an aldehyde group (-CHO) attached to a benzene ring. In this molecule, the oxygen atom in the hydroxyl group can form a hydrogen bond with the hydrogen atom attached to the carbon atom in the aldehyde group within the same molecule. This intramolecular hydrogen bond stabilizes the structure of salicylaldehyde.

Water (option B) does not exhibit intramolecular hydrogen bonding because it forms intermolecular hydrogen bonds between different water molecules.

Acetaldehyde (option C) does not exhibit intramolecular hydrogen bonding. It contains a carbonyl group (-CO-) but lacks any hydrogen bond donor or acceptor within the molecule.

Phenol (option D) exhibits intermolecular hydrogen bonding between different phenol molecules but does not have intramolecular hydrogen bonding. It contains a hydroxyl group (-OH) attached to a benzene ring.

Therefore, the correct answer is A. salicylaldehyde.