Which line is an example of apostrophe?

The correct option is B.
In poetry, apostrophe refers to the figure of speech, which the author use to address an absent person, an abstract idea or a thing. Sometimes, the apostrophe is represented by the exclamation 'O'.
In the question given above, the poet is addressing day and he is asking it that where is it going to find a cavern that is dark enough to mask its monstrous visage.