Final answer:
HD is not a real acid, while HCl, HF, and HI are recognized acids, with HCl being an example of a strong acid.
Step-by-step explanation:
The acid that is not real among the options provided is HD. Hydrochloric acid (HCl), hydrofluoric acid (HF), and hydroiodic acid (HI) are all well-known and recognized acids. However, HD is not recognized as a standard chemical formula for an acid. Acids such as HCl are known to be strong acids because they dissociate 100% into ions in water. An acid not listed among the common strong acids is generally considered a weak acid, regardless of its degree of ionization in a solution.