Which one of the following compounds shows the presence of intramolecular hydrogen bond?

Which of the following molecules shows intramolecular hydrogen bonding?

Which of the following compounds forms the maximum number of hydrogen bonds?

The density of water is higher than that of ice due to:

In which of the following compounds is intramolecular hydrogen bonding present?

Hydrogen bonding plays a central role in the following phenomena:
$(A)$ Ice floats in water.
$(B)$ Higher Lewis basicity of primary amines than tertiary amines in aqueous solutions.
$(C)$ Formic acid is more acidic than acetic acid.
$(D)$ Dimerisation of acetic acid in benzene.