A : When a steady current flows through a conductor of non - uniform cross-section , the current density , electric field and drift velcoity do not remain constant.
R : For a constant current the current density electric field and drift velocity are inversely proportional to cross - sectional area.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion , then mark
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion then mark.
C. If Assertion is ture statement but Reason is false , then mark
D. If both Assertion and Reason are false statements then mark.

Answer

To solve the question, we will analyze the assertion (A) and reason (R) provided, and determine their validity based on the principles of current electricity.

### Step-by-Step Solution:

**Step 1: Understand the Assertion (A)**
The assertion states that when a steady current flows through a conductor of non-uniform cross-section, the current density (J), electric field (E), and drift velocity (V_d) do not remain constant.

- **Current Density (J)** is defined as the current per unit area, \( J = \frac{I}{A} \).
- **Electric Field (E)** is related to current density and conductivity by Ohm's law: \( J = \sigma E \).
- **Drift Velocity (V_d)** is given by the equation \( I = n e A V_d \), where \( n \) is the number of charge carriers per unit volume and \( e \) is the charge of an electron.

**Step 2: Analyze the Reason (R)**
The reason states that for a constant current, the current density, electric field, and drift velocity are inversely proportional to the cross-sectional area (A).

- Since \( J = \frac{I}{A} \), if the current (I) is constant and the area (A) changes, then \( J \) will change inversely with \( A \).
- Similarly, from \( J = \sigma E \), if \( J \) changes, \( E \) must also change inversely with \( A \) (assuming \( \sigma \) is constant).
- From the equation \( I = n e A V_d \), if \( A \) changes and \( I \) is constant, \( V_d \) will also change inversely with \( A \).

**Step 3: Conclusion**
Since all three quantities (J, E, V_d) depend on the cross-sectional area and change inversely with it when the current is constant, the assertion is true. The reason correctly explains why the assertion is true.

Thus, both the assertion and the reason are true, and the reason is the correct explanation for the assertion.

### Final Answer:
Both assertion (A) and reason (R) are true, and the reason is the correct explanation for the assertion.