Correct option is A

Given:

• Two concentric rings made of metals A and B, with ring A having a larger diameter.
• The thermal expansion coefficients of metals A and B are $c$ (outer ring) and $c$ (inner ring), respectively.
• We need to determine the behavior of the gap between the rings under thermal expansion.

Thermal Expansion Concept:

The change in diameter of each ring due to thermal expansion is proportional to its thermal expansion coefficient. The behavior of the gap depends on how much each ring expands.

1. Case 1: $c<c$

   • The inner ring (B) expands more than the outer ring (A).
   • This causes the gap to decrease, as the inner ring grows closer to the outer ring.

2. Case 2: $c=c$

   • Both rings expand equally.
   • The gap remains the same, as the growth in their diameters is identical.

3. Case 3: $c>c$

   • The outer ring (A) expands more than the inner ring (B).
   • This causes the gap to increase, as the outer ring moves further away from the inner ring.

Correct Explanation:

Since the question specifies that $c<c$, the gap will decrease, which matches statement A.

Final Answer:

(a) Only A.