Final answer:
Percy, and other humans, are likely unable to see the creatures because they are invisible to humans, which is a common trope in mythology and fantasy stories where magical creatures exist in a realm beyond normal human perception. The correct option is b.
Step-by-step explanation:
The question pertains to why Percy hasn't been able to see certain creatures and the answer lies within the context of the storyline which usually involves fantastical elements. The most likely answer is that the creatures are invisible to humans, as is commonly found in mythology and fantasy narratives.
This option b) aligns with many stories where magical creatures are not perceived by the mundane human senses due to enchantments or a separate reality. Characters like Percy often discover that they have a special ability or destiny that allows them to perceive these hidden layers of the world, which normal humans cannot see.
This can be compared to the allegory described by Socrates to Glaucon, wherein the prisoners in the cave only see shadows and believe them to be reality because they have never perceived the real objects.
Similarly, Wordsworth's poem mourns the modern disconnection from nature and myth, wishing for a return to a time when people could see and believe in the mythical and divine in the natural world. The correct option is b.