The correct answer is external boundary.
A. surface area
B. shortest route
C. point in a planetary orbit
D. external boundary
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
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.