Which line is an example of apostrophe?

o "a cavern dark enough"

"Then by day"

"Seek none, conspiracy."

"Hide it"

The answer is your first option: o "a cavern dark enough"

Explanation:

Apostrophe in literature is a figure of speech and it basically consists on presenting an exclamation - -for instance in the correct answer we have "o" although with no "h"- - also, the writer or speaker talks to an inanimate thing or someone dead, which totally this is the case, as the inanimate object here is "the cavern". On the other hand, the other options don't give you neither the exclamation nor the talk to someone dead or inanimate thing.

Answer: Its actually option C.) on edge.