Concept:The correct prepositional phrase to describe someone wearing a uniform is "in uniform".Explanation:The phrase "in uniform" means wearing the official clothes of a particular job or organisation.It is the standard idiomatic expression used with officers, soldiers, police, and similar professionals."In uniform dress" is not the natural or correct phrase in this context."On uniform" and "with uniform" are grammatically incorrect usages.Thus, the completed sentence is: "The officer in uniform is my brother."Answer:C. in uniform