What does the prefix 'co-' mean?

• A. Together
• B. Between
• C. Three
• D. Not

Answer: (A)

The prefix co- means together, working as a unit or side by side. You can see it in words like cooperate (work together), coordinate (bring parts together to function as one), or coauthor (someone who writes something with another person). That sense of joint action or sharing is exactly what co- conveys, so “Together” is the best fit.

The other options map to different ideas. A prefix meaning between would be inter- (or a related form), a prefix meaning three is tri-, and a prefix meaning not is un- or non-.