Chemically heroin is

Morphine possesses two alcoholic −OH groups (at C-3 and C-6 positions) that can undergo acetylation.

When morphine is treated with acetic anhydride, both of these hydroxyl groups are esterified, giving the compound $$\text{CH}_3COO\!-\!\text{morphine}\!-\!OOCCH_3$$. Because two acetyl (CH3CO-) groups are introduced, the product is called diacetylmorphine.

Diacetylmorphine is the pharmaceutical (and illicit) drug commonly known as heroin. Thus, heroin is the diacetate ester of morphine.

Therefore, the correct choice is:
Option C which is: morphine diacetate