Bond line formula of $$HOCH(CN)_2$$ is :

In bond-line notation, carbon atoms are represented by vertices or line ends and are not written explicitly. Hydrogen atoms attached to carbon are also omitted. Only heteroatoms and hydrogens attached to them are shown.

For $$HOCH(CN)_2$$, the central carbon is bonded to one $$-OH$$ group, two $$-CN$$ groups, and one hydrogen atom.

The central carbon should therefore be represented only by a vertex. The $$-OH$$ and $$-CN$$ groups must be shown explicitly, while the hydrogen attached to the central carbon remains hidden.

Options A, B, and C are incorrect because they explicitly show the central carbon and/or its attached hydrogen, which violates the conventions of bond-line notation.

Option D is correct because the central carbon is represented by a vertex, the $$-OH$$ and two $$-CN$$ groups are shown explicitly, and the hydrogen attached to the central carbon is omitted as required.

Hence, the correct answer is Option D.