To solve the question regarding the deficiency that causes loss of appetite, mental confusion, fatigue, and muscle depreciation, we can follow these steps:
### Step-by-Step Solution:
1. **Identify the Symptoms**: The symptoms mentioned in the question include loss of appetite, mental confusion, fatigue, and muscle depreciation.
2. **Review the Options**: The options provided are riboflavin, vitamin C, vitamin K, and thiamine. We need to analyze each vitamin to see which one correlates with the symptoms listed.
3. **Analyze Riboflavin (Vitamin B2)**:
- Riboflavin deficiency can lead to symptoms like ulceration of the corners of the mouth (angular stomatitis) and skin diseases.
- It does not typically cause mental confusion, fatigue, or muscle depreciation.
4. **Analyze Vitamin C**:
- Vitamin C deficiency leads to scurvy, which is characterized by bleeding gums, bruising, and slow healing.
- It does not directly cause mental confusion or muscle depreciation.
5. **Analyze Vitamin K**:
- Vitamin K is essential for blood clotting. Its deficiency can lead to excessive bleeding and bruising.
- However, it is not associated with loss of appetite, mental confusion, or muscle depreciation.
6. **Analyze Thiamine (Vitamin B1)**:
- Thiamine deficiency is known to cause symptoms such as muscle mass loss, mental confusion, and fatigue.
- It is crucial for energy metabolism and nerve function. A deficiency can lead to conditions like Wernicke-Korsakoff syndrome, which includes confusion and muscle weakness.
7. **Conclusion**: Based on the analysis, thiamine (Vitamin B1) is the vitamin whose deficiency correlates with the symptoms of loss of appetite, mental confusion, fatigue, and muscle depreciation.
### Final Answer:
The correct answer is **Thiamine (Vitamin B1)**.
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