Rajasthan Police Constable Model Paper 7

Here, the $2^{\mathit{n}\mathit{d}}$ word denotes the body part while the $1^{\mathit{s}\mathit{t}}$ word is worn to cover that body part ( $2^{\mathit{n}\mathit{d}}$ word) i.e.
Trousers: Legs → Trousers is worn in Legs.
Similarly, Cap is worn on head to cover it.
Hence, Head is the correct alternative.