Final answer:
The phrase in question uses a metaphor to directly compare 'shadows' with 'riddles' and 'unseen stars', which is a figurative device that implies a likeness without using 'like' or 'as'. Therefore the correct answer is A. Metaphor.
Step-by-step explanation:
The statement 'Shadows indicate riddles illuminating unseen stars' does not provide a direct comparison using 'like' or 'as', which would indicate a simile, nor does it contain an exaggeration for effect, which would suggest hyperbole, nor does it present an outcome that is the opposite of what is expected, which would suggest irony.
Instead, it directly compares 'shadows' with 'riddles' and 'unseen stars', suggesting that the former signifies or metaphorically reveals the latter.
This is an example of a metaphor, a figurative language device where one thing is spoken of as though it were something else, thereby implying a likeness or analogy between them. In the context of the Allegory of the Cave, the metaphor is a central mechanism through which Plato explores the themes of perception, reality, and knowledge.
The prisoners in the cave mistake shadows for reality, but those shadows are just copies of the real objects illuminated by the light, representing a deeper truth they cannot perceive.