The most important active step in the development of periodic table was taken by

Options

A. Mendeleev
B. Dalton
C. Avogadro
D. Cavendish

Answer

To answer the question regarding who took the most important active step in the development of the periodic table, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the individual who made the most significant contribution to the development of the periodic table.

2. **Identifying Key Figures**: Several scientists contributed to the development of the periodic table, including John Dalton, Amedeo Avogadro, and Henry Cavendish. However, we need to focus on the one who made the most pivotal change.

3. **Reviewing Contributions**:
- **John Dalton**: Proposed the atomic theory but did not organize the elements into a periodic table.
- **Amedeo Avogadro**: Known for Avogadro's law, which relates to the volume of gases but did not directly contribute to the periodic table.
- **Henry Cavendish**: Conducted experiments in chemistry but did not contribute to the periodic table's development.

4. **Recognizing Dmitri Mendeleev's Contribution**: The most significant step in the development of the periodic table was taken by Dmitri Mendeleev. He organized the elements based on atomic mass and properties, leading to the formulation of the periodic law.

5. **Final Answer**: Therefore, the most important active step in the development of the periodic table was taken by **Dmitri Mendeleev**.