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.
Here are the answers to the given sentences above:-No one answered the ad but me. It is used as PREPOSITION.-There is but one choice to make in this case. It is used as an ADVERB.-Sadie said she would go but she had to bring her sister. It is used as a CONJUNCTION