Which prefix means 'Out'?

• A. Exo-
• B. Ecto-
• C. Extra-
• D. Contra-

Answer: (A)

Outward or exterior position is what this prefix conveys. Exo- directly signals that something is on the outside or external to its body or structure, and it forms common terms like exoskeleton (the outer skeleton), exocrine glands (secreted to the outside), or exogenous substances (originating from outside). Ecto- also means outside, but it’s used more in contexts describing the external surface or parts of organisms (such as ectoderm, the outer embryonic layer). Extra- indicates outside or beyond in a broader sense, but it’s less about the immediate outward position and more about being beyond a boundary (as in extracellular, exterior). Contra- means against. So exo- is the best match for the simple idea of “out.”