The law of constant proportion was proposed by

Options

A. Proust
B. Einstein
C. Richter
D. Dalton

Explanation

**Step-by-Step Solution:**

1. **Identify the Law of Constant Proportion**: The law states that a chemical compound always contains its component elements in fixed ratio by mass, regardless of how the compound was prepared or the source from which it was obtained.

2. **Determine the Proposer of the Law**: The law of constant proportion was proposed by Joseph Proust.

3. **Year of Proposal**: Joseph Proust proposed this law in the year 1797.

4. **Explain the Law with an Example**: For instance, water (H2O) is a compound that can be obtained from various sources such as rivers, oceans, or through chemical reactions. Regardless of the source, the composition of water remains the same: it always consists of two hydrogen atoms and one oxygen atom.

5. **Summarize the Key Points**:
- The law of constant proportion is fundamental in chemistry.
- It emphasizes that the composition of a compound is invariant, regardless of the method of preparation or source.

6. **Final Answer**: The law of constant proportion was proposed by Joseph Proust.

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