Assertion: Cyclotron does not accelerate.
Reason: Mass of the electron is very small.

Options

A. Both assertin and reason are true and reason is the correct explanation of assertion
B. Both assertion and reason are true but reason is not the correct explanation of assertion
C. Assertion is true but reason is flase
D. Both assertion and reason are false

Explanation

To solve the question, we need to analyze the assertion and reason provided:

**Assertion:** Cyclotron does not accelerate.
**Reason:** Mass of the electron is very small.

### Step-by-Step Solution:

1. **Understanding the Cyclotron:**
- A cyclotron is a type of particle accelerator that uses a magnetic field and an oscillating electric field to accelerate charged particles, such as electrons, to high speeds.

2. **Analyzing the Assertion:**
- The assertion states that "Cyclotron does not accelerate." This is somewhat misleading. A cyclotron can accelerate particles, but it is not effective for very light particles like electrons due to relativistic effects.

3. **Analyzing the Reason:**
- The reason states that "Mass of the electron is very small." This is true; the mass of an electron is approximately \(9.11 \times 10^{-31}\) kg, which is significantly smaller than that of protons or heavier ions.

4. **Relativistic Effects:**
- As particles are accelerated to speeds close to the speed of light (denoted as \(c\)), their effective mass increases due to relativistic effects. This is given by the equation:
\[ m = \frac{m_0}{\sqrt{1 - \frac{v^2}{c^2}}} \]
where \(m_0\) is the rest mass and \(v\) is the velocity of the particle.

5. **Impact on Cyclotron Frequency:**
- The cyclotron frequency (the frequency at which a charged particle orbits in a magnetic field) is given by:
\[ f = \frac{qB}{2\pi m} \]
where \(q\) is the charge, \(B\) is the magnetic field strength, and \(m\) is the mass of the particle.
- As the speed of the electron increases, its effective mass increases, which leads to a decrease in cyclotron frequency.

6. **Conclusion on Assertion and Reason:**
- Due to the small mass of the electron, it can reach relativistic speeds very quickly. As it does so, its mass increases, causing it to fall out of sync with the oscillating electric field in the cyclotron. This prevents the cyclotron from effectively accelerating electrons.
- Therefore, both the assertion and the reason are correct, and the reason correctly explains the assertion.

### Final Answer:
Both the assertion and the reason are true, and the reason correctly explains the assertion.