(A) : Ampere's law holds for steady currents in straight conductor which do not vary with time.
(R) : Magnetic field lines always form closed loops.

Options

A. Both 'A' and 'R' are true and 'R' is the correct explanation of 'A'.
B. Both 'A' and 'R' are true and 'R' is not the correct explanation of 'A'
C. 'A' is true and 'R' is false
D. 'A' is false and 'R' is false

Answer

To solve the problem, we need to analyze the two statements given:

**Statement (A)**: Ampere's law holds for steady currents in straight conductors which do not vary with time.

**Statement (R)**: Magnetic field lines always form closed loops.

### Step-by-Step Solution:

1. **Understanding Ampere's Law**:
- Ampere's law states that the line integral of the magnetic field \( B \) around a closed loop is proportional to the electric current \( I \) passing through the loop. Mathematically, it is expressed as:
\[ \oint B \cdot dl = \mu_0 I \]
- This law is valid for steady currents, meaning the current does not change with time.

2. **Analyzing Statement (A)**:
- The statement correctly asserts that Ampere's law applies to steady currents in straight conductors. When current is constant (not varying with time), we can use Ampere's law to find the magnetic field around the conductor.

3. **Understanding Magnetic Field Lines**:
- Magnetic field lines represent the direction and strength of the magnetic field. They always form closed loops, meaning they do not begin or end but rather loop back on themselves.
- This is a fundamental property of magnetic fields, as they emerge from the north pole of a magnet and enter at the south pole.

4. **Analyzing Statement (R)**:
- The statement is also true; magnetic field lines indeed always form closed loops. However, this property of magnetic field lines does not directly explain why Ampere's law is valid for steady currents.

5. **Conclusion**:
- Both statements (A) and (R) are true. However, the reason (R) does not provide a correct explanation for the assertion (A). The validity of Ampere's law for steady currents is based on the constancy of the current, not on the nature of magnetic field lines.

### Final Answer:
Both assertion and reason are true, but the reason is not the correct explanation of the assertion.