The most important active step in the development of periodic table was taken by
Mendeleev
Dalton
Avogadro
Cavendish.

Options

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

Answer

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

### Step 1: Understand the Contributions of Each Scientist
- **Mendeleev**: Known for creating the first widely recognized periodic table, arranging elements by atomic mass and predicting the properties of undiscovered elements.
- **Dalton**: Proposed the atomic theory, which laid the groundwork for modern chemistry but did not directly contribute to the periodic table.
- **Avogadro**: Known for Avogadro's law regarding gas volumes and the concept of mole, but he did not contribute to the periodic table.
- **Cavendish**: Conducted experiments on gases and is known for discovering hydrogen, but he did not contribute to the periodic table.

### Step 2: Identify the Key Figure
From the contributions listed, it is clear that **Mendeleev** is the scientist who took the most significant step in the development of the periodic table.

### Step 3: Select the Correct Answer
Based on the analysis, the correct answer to the question is **Mendeleev**.

### Final Answer
The most important active step in the development of the periodic table was taken by **Mendeleev**.

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