Assertion : A beam of protons is moving towards east in vertically upward magnetic field. Then, this beam will deflect towards south.
Reason : A constant magnetic force will act on the proton beam.

Options

A. If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.
B. If both Assertion and Reason are true but Reason is not the correct explanation of Assertion
C. If Assertion is true, but the Reason is false.
D. If Assertion is false but the Reason is true.

Explanation

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

### Step 1: Analyze the Assertion
The assertion states that a beam of protons is moving towards the east in a vertically upward magnetic field, and it will deflect towards the south.

- **Direction of Motion**: The protons are moving towards the east.
- **Direction of Magnetic Field**: The magnetic field is directed vertically upward.

### Step 2: Apply the Right-Hand Rule
To determine the direction of the magnetic force acting on the protons, we can use the right-hand rule for the cross product of velocity and magnetic field.

- **Right-Hand Rule**: Point your fingers in the direction of the velocity (east), curl them in the direction of the magnetic field (upward), and your thumb will point in the direction of the magnetic force.

### Step 3: Determine the Direction of the Force
Using the right-hand rule:
- Fingers point east (direction of velocity).
- Curl fingers upward (direction of magnetic field).
- The thumb will point towards the south.

Thus, the assertion is correct: the beam will deflect towards the south.

### Step 4: Analyze the Reason
The reason states that a constant magnetic force will act on the proton beam.

- While it is true that a magnetic force acts on charged particles moving in a magnetic field, the nature of this force is such that it depends on the velocity of the protons and the magnetic field.
- As the protons move, their direction changes, which means the angle between the velocity vector and the magnetic field vector changes, leading to a change in the direction of the magnetic force.

### Step 5: Conclusion
- The assertion is true: the beam will deflect towards the south.
- The reason is false: the magnetic force is not constant in direction as the velocity changes.

### Final Answer
- Assertion: True
- Reason: False