The prefix ecto/exo- means which of the following?

• A. Inner
• B. Across
• C. Outside
• D. Outer

Answer: (D)

Ecto- and exo- signal location on the outside of something. They describe structures or origins that are external, on the exterior rather than the interior. That’s why terms like ectoderm or exoskeleton refer to outside surfaces or outer layers. Among the given options, the best fit is outer because it directly captures the idea of being on the exterior or the external side. Inner describes the opposite, located inside. Across is about movement or relation through a boundary, not specifically about external location, and outside is close, but outer is the precise positional descriptor used to talk about structures on the exterior.