What explains the eastward flow of the equatorial counter-current?

• (a) The Earth's rotation on its axis — Coriolis force affects ocean currents generally, but it does not specifically cause the equatorial counter-current to flow eastward. Incorrect.
• (b) Convergence of the two equatorial currents — The North and South Equatorial Currents do help create a return flow, but this is not the standard UPSC explanation for its eastward flow. Not the best answer.
• (c) Difference in salinity of water — Salinity contrast is not the controlling factor behind the equatorial counter-current. Incorrect.
• (d) Occurrence of the belt of calm near the equator — In the doldrum belt/ITCZ, winds are weak, so the westward-driving trade winds lose strength; this allows water piled up in the west to move back eastward as the equatorial counter-current. Correct.

Updated conclusion: UPSC treats the belt of calm near the equator as the key explanation, so (d) is the correct answer. Note: Option (b) describes a related contributing process, but in exam terms (d) is the accepted answer.