The black color of copper oxide turns blue on treating with HCl because of:
(A) Formation of cupric chloride
(B) Formation of water
(C) Release of hydrogen
(D) Release of carbon dioxide

To solve the question, let's analyze the chemical reaction between copper oxide (CuO) and hydrochloric acid (HCl).

### Explanation:
1. **Copper Oxide (CuO)**: This is a black solid that is a basic oxide of copper.
2. **Hydrochloric Acid (HCl)**: This is a strong acid that, when reacted with metal oxides, can lead to the formation of metal salts.

When copper oxide is treated with hydrochloric acid, the following reaction occurs:

\[
\text{CuO} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O}
\]

In this reaction:
- **CuO** (black) reacts with **HCl** to form **CuCl₂** (cupric chloride), which is blue in color, and water (**H₂O**).

### Analysis of Options:
- **(A) Formation of cupric chloride**: Correct. Cupric chloride (CuCl₂) is blue in color and is formed in the reaction.
- **(B) Formation of water**: While water is indeed produced in the reaction, it is not the main reason for the color change from black to blue.
- **(C) Release of hydrogen**: There is no hydrogen gas released in this reaction.
- **(D) Release of carbon dioxide**: Carbon dioxide is not involved in this reaction.

### Conclusion:
The correct answer is **(A) Formation of cupric chloride**, as the black copper oxide turns blue due to the formation of the blue cupric chloride in the reaction with hydrochloric acid.