what is an essential constituent of all organic substances.

Explanation

**Step-by-Step Solution:**

1. **Understanding Organic Substances**: Organic substances are compounds that primarily contain carbon atoms. They are the basis of life and are found in all living organisms.

2. **Identifying the Essential Constituent**: The question asks for the essential constituent of all organic substances. The term "essential constituent" refers to the fundamental element that is crucial for the formation of these substances.

3. **Recognizing the Role of Carbon**: Carbon is the key element in organic chemistry. It has unique properties that allow it to form stable bonds with many other elements, including itself. This property is known as "self-linking" or catenation.

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