Which one of the following is considered as the main postulate of Bohr's model of atom.

Options

A. Protons are present in the nucleus
B. Electrons are revolving around the nucleus
C. Centrifugal force produced due to the revolving electrons balance the force of attraction between the electron and the protons
D. Angular momentum of electron is an integral multiple of `h/(2pi)`

Answer

To solve the question regarding the main postulate of Bohr's model of the atom, we will analyze the options given and identify the correct one based on the principles of Bohr's model.

### Step-by-Step Solution:

1. **Understanding Bohr's Model**:
- Bohr's model of the atom was proposed by Niels Bohr in 1913. It describes the behavior of electrons in atoms, particularly hydrogen.

2. **Identifying the Postulates**:
- The main postulates of Bohr's model include:
- Electrons revolve around the nucleus in fixed orbits or shells.
- Each orbit corresponds to a specific energy level.
- The angular momentum of an electron in these orbits is quantized and is given by the formula:
\[ L = n \frac{h}{2\pi} \]
where \( L \) is the angular momentum, \( n \) is a positive integer (1, 2, 3,...), and \( h \) is Planck's constant.
- Electrons can absorb or emit energy when they transition between these orbits.

3. **Analyzing the Options**:
- **Option A**: Protons are present in the nucleus.
- This is true but not a postulate of Bohr's model.
- **Option B**: Electrons revolve around the nucleus.
- This is a description of electron behavior but not the main postulate.
- **Option C**: Centripetal force produced due to the revolving electron balances the force of attraction between the electron and proton.
- This describes a condition in Bohr's model but is not a postulate.
- **Option D**: The angular momentum of the electron is an integral multiple of \( \frac{h}{2\pi} \).
- This is indeed one of the main postulates of Bohr's model.

4. **Conclusion**:
- Based on the analysis, the correct answer is **Option D**: The angular momentum of the electron is an integral multiple of \( \frac{h}{2\pi} \).

### Final Answer:
**D**: The angular momentum of the electron is an integral multiple of \( \frac{h}{2\pi} \).