Smack is chemically:

The correct option is Diacetyl morphine.

Heroin, commonly known as "smack", is a semi-synthetic opioid drug.
It is chemically synthesized by the acetylation of morphine, a natural alkaloid extracted from the latex of the opium poppy plant, Papaver somniferum.

During this chemical reaction, two acetyl groups are added to the morphine molecule, resulting in the compound diacetylmorphine (or diacetyl morphine).
Chemically, it is represented as:
$$\text{Morphine}+2{\text{CH}}_3\text{CO-}\to \text{Diacetylmorphine (Smack)}$$

Therefore, smack is chemically diacetyl morphine.