**Step-by-Step Solution:**
1. **Understand the Question**: The question asks about the origin of excretory organs. Specifically, we need to identify which germ layer gives rise to these organs.
2. **Identify the Germ Layers**: After implantation, the inner cell mass of the blastocyst differentiates into three primary germ layers:
- Ectoderm (outer layer)
- Mesoderm (middle layer)
- Endoderm (inner layer)
3. **Function of Each Germ Layer**:
- **Ectoderm**: This layer forms structures such as the skin, brain, spinal cord, nerves, ears, nails, and hair.
- **Endoderm**: This layer gives rise to internal organs such as the lungs and pancreas.
- **Mesoderm**: This layer is responsible for developing structures such as the heart, kidneys, gonads, genital ducts, cartilage, muscles, blood, and other connective tissues.
4. **Focus on Excretory Organs**: The primary excretory organs in humans are the kidneys. To determine the origin of the kidneys, we refer to the mesoderm.
5. **Conclusion**: Since the kidneys, which are excretory organs, originate from the mesoderm, the correct answer to the question is option B (Mesoderm).
**Final Answer**: The excretory organs originate from the Mesoderm (Option B).
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