He was hiding ___ the curtain.

The correct preposition is Behind. When someone is hiding, they are typically concealed from view, and "behind" indicates being at the back of something, obscured by it.

• Below and Beneath mean at a lower level than something. Hiding below or beneath a curtain would imply being on the floor under it, which doesn't suggest effective concealment by the curtain itself.
• Under also means directly below, often implying being covered by something. While one might hide under a bed or a blanket, hiding under a curtain doesn't convey the intended meaning of being concealed by its vertical expanse.