In the context of mitigating the impending global warming due to anthropogenic emissions of carbon dioxide, which of the following can be potential sites for carbon sequestration?1.Abandoned and uneconomic coal seams2.Depleted oil and gas reservoirs3.Subterranean deep saline formationsSelect the correct answer using the code given below:

1. A

   1 and 2 only

   (a) 1 and 2 only. Abandoned coal seams and depleted hydrocarbon reservoirs are valid CO2 sinks, but deep saline formations are also standard sequestration geology. Omitting 3 is incomplete.

2. B

   3 only

   (b) 3 only. Saline aquifers work, but they are not the only class. Not the key.

3. C

   1 and 3 only

   (c) 1 and 3 only. Oil and gas reservoirs are a major enhanced-oil-recovery / storage option. Dropping 2 is wrong.

4. D

   1, 2 and 3

   (d) 1, 2 and 3. Uneconomic coal seams, depleted oil and gas fields, and deep saline formations are all recognised geological stores for captured CO2. This is the official key.

Summary. Official key is (d) 1, 2 and 3. Geological carbon sequestration injects compressed CO2 into porous rock isolated by caprock. Unmineable coal can adsorb CO2; depleted oil and gas reservoirs have known seals and may yield extra hydrocarbons; deep saline formations offer the largest pore volume. Forests and soils are biological sinks, not asked here. All three listed sites count.