Which of the following is a semi-deponent verb?

The term "semi-deponent" refers to a specific category of verbs in Latin that have a mixed conjugation pattern. These verbs are primarily active in meaning but have certain forms that are passive in construction, typically in the perfect system.

"Audeo" fits this definition perfectly. This verb means "to dare" and behaves like a deponent verb in the perfect tense, using passive forms while being active in meaning. For example, the perfect forms of "audeo" do not have an active voice but convey an active meaning, which exemplifies its semi-deponent status.

In contrast, "praesidium" is actually a noun meaning "protection" or "garrison," and does not fall under the category of a verb. "Memini," meaning "I remember," is classified as a deponent verb but not a semi-deponent, as it has no active forms at all. Lastly, "coetus," which refers to a "meeting" or "assembly," is also a noun and does not display any of the properties associated with semi-deponent verbs.

Thus, "audeo" is the correct answer since it exemplifies the characteristics of a semi-deponent verb in Latin.