**Step-by-Step Solution:**
1. **Understanding Deoxycytidine**:
- Deoxycytidine is a specific type of molecule that is part of DNA. It is classified as a nucleoside.
2. **Defining Nucleoside**:
- A nucleoside consists of two main components: a pentose sugar and a nitrogenous base. In the case of deoxycytidine, the sugar is deoxyribose, and the nitrogenous base is cytosine.
3. **Identifying the Components**:
- In deoxycytidine:
- The pentose sugar is deoxyribose.
- The nitrogenous base is cytosine.
4. **Analyzing the Options**:
- The question provides several options regarding what deoxycytidine is:
- **Option A**: Nucleoside of DNA
- **Option B**: Nucleoside of RNA
- **Option C**: Nucleotide of DNA
- **Option D**: Nucleotide of RNA
5. **Evaluating Each Option**:
- **Option A**: Nucleoside of DNA - This is correct because deoxycytidine is indeed a nucleoside found in DNA.
- **Option B**: Nucleoside of RNA - This is incorrect because deoxycytidine is not found in RNA; instead, RNA contains ribonucleosides.
- **Option C**: Nucleotide of DNA - This is incorrect because deoxycytidine is a nucleoside, not a nucleotide. A nucleotide includes a phosphate group in addition to the sugar and base.
- **Option D**: Nucleotide of RNA - This is incorrect for the same reason as Option C; deoxycytidine is not a nucleotide.
6. **Conclusion**:
- The correct answer is **Option A: Nucleoside of DNA**.
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