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

The North and South Equatorial Currents transport large volumes of water westward, causing it to pile up against the western boundaries of ocean basins; the resulting pressure gradient forces water to flow back eastward as the Equatorial Counter-Current. Option (a) is a common distractor because while Earth's rotation influences the Coriolis force, the counter-current is primarily a gravity-driven response to this physical accumulation of water rather than a direct result of rotation. This question tests the core concept of ocean current dynamics and the formation of sea-level gradients.