While all four words describe a lack of ornamentation or complexity, each carries a slightly different tone. 'Plain' (B1) is the most neutral and common, used for anything without special features (a plain T-shirt, plain yogurt). 'Simple' (A2) is broader, focusing on ease or lack of complexity rather than visual decoration. 'Bare' (B2) suggests the minimum possible — often uncomfortably or strikingly so (a bare room with no furniture). 'Stark' (C1) adds a sense of severity or harshness (a stark white hospital corridor). 'Unadorned' specifically means 'without any decorative additions' and often implies a deliberate, aesthetic choice. For example, 'She wore a plain dress' means it had no pattern, but 'She wore an unadorned dress' emphasizes the intentional absence of embellishment like lace or embroidery.
Formed by adding the prefix 'un-' (meaning 'not', from Old English) to 'adorned', the past participle of 'adorn'. The verb 'adorn' comes from Old French 'adorner', which traces back to Latin 'adornare' — a combination of 'ad-' (to) and 'ornare' (to equip, decorate, or adorn).