The flat, fine-grained bed of a temporary (ephemeral) lake in a desert basin is known as a:

A playa is a flat, dry lake bed in the lowest part of a desert basin. It is covered with fine-grained sediments such as clay, silt and salts, and holds water only for short periods after rain.

How playas form:

• In deserts, there is often no drainage to the sea (inland drainage).
• After occasional heavy rain, streams carry water and fine sediment into the lowest part of the basin, forming a shallow temporary lake.
• Because of the intense heat, the water evaporates quickly, leaving behind a flat layer of clay, silt and salt.
• Over time, these deposits build up a very flat surface, one of the flattest natural surfaces on Earth.

Features:

• When dry, the surface often cracks into polygon-shaped patterns.
• If the water is salty, a white crust of salt forms; such playas are called salinas or salt flats.
• The term "playa" comes from Spanish, meaning "beach".

Examples:

• The Bonneville Salt Flats in Utah, USA, used for land-speed record attempts.
• Racetrack Playa in Death Valley, USA, famous for its mysterious "sailing stones" that move across the surface.
• The Salar de Uyuni in Bolivia, the world's largest salt flat.
• In India, the salt lakes of Rajasthan, such as the Sambhar, Didwana and Pachpadra, and the Rann of Kutch in Gujarat share some playa-like features; in Rajasthan, such saline depressions are often called "dhands".

Other desert landforms:

• Oasis: a fertile spot in a desert, where groundwater reaches the surface.
• Wadi: a dry, steep-sided valley or river bed in a desert that carries water only after rain.
• Bajada: a gently sloping surface formed by the joining of several alluvial fans at the foot of mountains.
• Pediment: a gently sloping rock surface at the base of a mountain.

Why not the other options?

• ✗An oasis is a fertile area with water in a desert.
• ✗A wadi is a dry stream bed.
• ✗A bajada is a sloping surface of merged alluvial fans.