Barkhan is a

Answer: (a) Crescent shaped sand-dune

A barkhan is a crescent-shaped dune with horns pointing downwind.

The idea

Wind-built landforms depend on wind direction, available sand and surface cover. A steady wind with limited loose sand can build a crescent dune on firmer ground. Identify the dune by its plan shape and the direction its tips point.

Wind moves loose sand → Sand gathers around a small obstacle → Gentle windward slope builds → Sand slips down the steep sheltered face → Thin edges move faster → Crescent horns extend downwind

Why the others are wrong

(b) That is a bar/barrier island.

(c) That describes basalt.

Also know

Barchan and barkhan are spellings of the same dune type. Parabolic dunes also curve, but their vegetation-held horns point upwind. Transverse dunes form ridges across the prevailing wind; longitudinal dunes extend broadly along the dominant wind direction. Mushroom rocks and yardangs are mainly erosional wind features, unlike dunes.