My brother is a ___.

The correct choice is 'lieutenant' because it is the accurate spelling of the military rank, which refers to an officer holding a position of authority in armed forces, police, and other organizations. The spelling of 'lieutenant' can vary between American and British English, but 'lieutenant' is widely recognized and accepted in both contexts as the proper form.

The other versions presented in the choices are incorrect due to misspellings that could lead to confusion or misunderstanding. 'Liuetenant,' 'liutenant,' and 'leutenant' do not correspond to any recognized word or rank in English. Proper spelling is crucial in professional and academic contexts, as it reflects attention to detail and the understanding of language conventions. Thus, 'lieutenant' is the only viable option among the choices provided.