Which of the following is not favourable for the formation of deltas?

Answer: (b) Currents and tides

Strong currents and tides wash sediment away — so no delta forms.

The idea

A delta grows when a river supplies more sediment than coastal processes remove. Quiet, shallow receiving water lets particles settle near the mouth. Waves, currents and tides can redistribute the load and prevent it from building a protruding delta.

River carries sediment towards the coast → Flow slows on entering the sea → Sediment begins to settle → Coastal currents and tides redistribute some sediment → A delta grows only if deposition exceeds removal

Why the others are wrong

(a) Calm conditions help deltas.

(c) Sheltered coasts help deltas.

(d) No lakes means the river keeps its sediment — helpful.

Elimination technique

Calm winds and shelter favour settling; a large upstream lake would trap sediment, so its absence can preserve supply. The option describing active marine removal is the unfavourable one.

Also know

River-dominated, wave-dominated and tide-dominated deltas have different shapes; strong tides do not make every delta impossible. Lakes and reservoirs upstream trap sediment and can reduce delivery to the coast. The Ganga-Brahmaputra system builds a delta, while the Narmada has an estuarine mouth. An estuary is a coastal water body where river water mixes with seawater.