Oxymoron means that two contradictory words or concepts are combined in a saying or sentence to give an effect. A common oxymoron is "jumbo shrimp." Everyone knows that shrimp can't be jumbo, but saying it like this lets us know that the shrimp is rather big.
The choice that has an oxymoron is:
D. I stood alone in the deafening silence.
Silence can not physically be deafening, as there is no noise at all. "Deafening silence" helps convey that it was so quiet that the person could have been deaf OR they could have meant that it was so quiet that it seemed loud.
The theme is basically brains over brawn, because the mouse continues to use his intelligence to escape the big and strong creatures that are after him, despite the mouse himself being small and frail.
