The correct answer is (D) The external boundary (of a surface, area, or body); the line bounding something, particularly something rounded; the outermost border, limit, or edge of something.

Periphery comes from Middle English periferie, Latin periferia, Greek periphéreia (circumference).

Examples of use:

• Sentries patrolled the periphery of the camp at all hours.
• The city's commercial section is located at the periphery of residential area.