Assertion: If an electron, while coming vertically from outerspace, enter the earth's magnetic field, it is deflected towards west.
Reason: Electron has negative charge.

Options

A. If both the assertion and reason are true and reason is a true explantion of the assertion.
B. If both the assertion and reason are true but the reason is not true the correct explantion of the assertion.
C. If the assertion is true but reason false
D. If both the assertion and reason are false.

Explanation

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

### Step 1: Understand the Assertion
The assertion states that an electron entering the Earth's magnetic field from outer space is deflected towards the west.

### Step 2: Determine the Direction of the Earth's Magnetic Field
The Earth's magnetic field can be visualized as having a direction: it generally points from the geographic south to the geographic north. For our purposes, we can assume:
- North is in the positive y-direction.
- South is in the negative y-direction.
- East is in the positive x-direction.
- West is in the negative x-direction.
- The vertical direction (where the electron is coming from) is along the z-axis.

### Step 3: Apply the Right-Hand Rule
To determine the direction of the force acting on the electron, we can use the right-hand rule:
1. Point your thumb in the direction of the velocity of the electron (which is downwards, along the negative z-axis).
2. Point your fingers in the direction of the magnetic field (which is directed towards the north, along the positive y-axis).
3. The palm of your hand will then point in the direction of the force acting on a positive charge. However, since the electron has a negative charge, the force will be in the opposite direction.

### Step 4: Calculate the Force on the Electron
Using the formula for the magnetic force:
\[ \vec{F} = q(\vec{v} \times \vec{B}) \]
Where:
- \( q \) is the charge of the electron (negative),
- \( \vec{v} \) is the velocity vector (downward),
- \( \vec{B} \) is the magnetic field vector (northward).

Given:
- \( q = -e \) (where \( e \) is the elementary charge),
- \( \vec{v} = -k \) (downward),
- \( \vec{B} = j \) (northward).

Using the cross product:
\[ \vec{F} = -e(-k \times j) \]
The cross product \( -k \times j \) gives a vector pointing towards the west (negative x-direction).

### Step 5: Conclusion on Assertion
Since the force on the electron is directed towards the west, the assertion is true.

### Step 6: Analyze the Reason
The reason states that the electron has a negative charge. While this is true, it does not explain why the electron is deflected towards the west. The deflection is a result of the interaction between the velocity of the electron and the Earth's magnetic field, not merely due to the charge being negative.

### Final Conclusion
- **Assertion**: True (the electron is deflected towards the west).
- **Reason**: True (the electron has a negative charge), but it is not the correct explanation for the assertion.

### Summary
- Assertion is true.
- Reason is true but not the correct explanation.