Choice "D" is correct. A power of attorney is a form of agency, and like all agencies the power of attorney agency relationship may be limited in scope of authority.
Choice "a" is incorrect. As a general rule, no writing is required to form an agency relationship, and even where a writing is required, the writing need only be signed by the principal (the one sought to be bound).
Choice "c" is incorrect. A power of attorney, like other agencies, need not explicitly state a duration.
Choice "b" is incorrect. A power of attorney, like all agencies, is terminated by the death of either the principal or the agent.