Final answer:
The phrase 'vacancy of expression' is a metaphor because it compares the woman's lack of emotional or thoughtful expression to an empty space without using 'like' or 'as.' Option b.
Step-by-step explanation:
The phrase "vacancy of expression" to convey the emptiness of the woman's memories is a metaphor. A metaphor is a figure of speech that directly compares one thing to another for rhetorical effect.
By describing the woman's expression as being 'vacant,' the author suggests a comparison to a uninhabited or empty space, thereby implying that her expression is devoid of emotion or thought, just like an empty room lacks contents.
This literary device does not use 'like' or 'as,' which would make it a simile, nor does it involve the repetition of sounds as in alliteration or the attribution of human qualities to non-human entities as in personification. Option b.