In fruit trees a disease 'exanthema' is caused by the deficiency of :

Options

A. Na
B. Ca
C. Cu
D. P

Answer

### Step-by-Step Solution:

1. **Understanding the Disease**: The question asks about the disease 'exanthema' in fruit trees. We need to identify what causes this disease.

2. **Identifying the Deficiency**: The options provided are sodium, calcium, copper, and phosphorus. We need to determine which mineral deficiency is linked to the disease.

3. **Researching Exanthema**: Exanthema is known to be associated with certain deficiencies in plants. Specifically, it is commonly linked to a deficiency of copper.

4. **Effects of Copper Deficiency**: When there is a deficiency of copper in fruit trees, it leads to symptoms such as:
- Yield of gums on the bark.
- Affects the fruit-bearing capacity.
- Dying of leaves, which is a major characteristic of this disease.

5. **Conclusion**: Based on the information gathered, the deficiency that causes the disease 'exanthema' in fruit trees is copper.

6. **Final Answer**: Therefore, the correct option is **c) copper**.