Out of the halogen acids, HX, the one with the highest degree of ionization is...............

Explanation

To determine which halogen acid (HX) has the highest degree of ionization, we need to analyze the four common halogen acids: HF (hydrofluoric acid), HCl (hydrochloric acid), HBr (hydrogen bromide), and HI (hydrogen iodide).

### Step-by-Step Solution:

1. **Understanding Halogen Acids**:
- The halogen acids are HF, HCl, HBr, and HI. These acids are formed from halogens and hydrogen.

2. **Definition of Degree of Ionization**:
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