Explanation
- A
The Earth's rotation on its axis
(a) The Earth's rotation on its axis. Rotation supplies Coriolis, which shapes gyres, but it does not by itself explain why a narrow eastward countercurrent sits on the equator. Rotation is background, not the keyed mechanism.
- B
Convergence of the two equatorial currents
(b) Convergence of the two equatorial currents. North and South Equatorial Currents are westward; their meeting is not the textbook reason UPSC keyed for the eastward countercurrent.
- C
Difference in salinity of water
(c) Difference in salinity of water. Salinity gradients can drive local thermohaline flow, but they are not the standard explanation of the Equatorial Counter Current.
- D
Occurrence of the belt of calm near the equator
(d) Occurrence of the belt of calm near the equator. Trade winds pile water westward; in the doldrums (equatorial belt of calms) that head of water returns east as the Equatorial Counter Current. The calm belt is the official key.
Summary. Official key is (d) Occurrence of the belt of calm near the equator. Westward trades stack water in the western ocean. Across the equatorial doldrums the piled water flows back east as the countercurrent. Rotation, current convergence and salinity are not the keyed explanation on this paper. The doldrums answer is the one that matches the official letter.