Assertion (A) : Coal-based thermal power stations contribute to acid-rain. Reason (R) : Oxides of carbon are emitted when the coal burns.

The correct answer is Option 2.

Assertion (A) is true because coal contains sulfur and nitrogen impurities. When burned in thermal power stations, these react with oxygen to form Sulfur Dioxide (SO₂) and Nitrogen Oxides (NOₓ). These gases react with atmospheric water vapor to form sulfuric and nitric acids, leading to acid rain.

Reason (R) is also true because coal is primarily composed of carbon; its combustion inevitably releases Oxides of Carbon (CO and CO₂). However, carbon oxides are primarily greenhouse gases and are not the principal precursors of acid rain (which is caused by SO₂ and NOₓ).

Since both statements are factually correct but the presence of carbon oxides does not explain the mechanism of acid rain formation, R is not the correct explanation of A. Therefore, Option 2 is the most accurate choice.