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In the excerpt, the word "pungent" refers to the pickles' aroma.
Pungent is a word to describe a very strong smell, very sharp and biting. So when Untermeyer used the word pungent to refer to the pickles, he was talking about their aroma, which is a fancy way to say smell.
If the root hydro- means "water", then the word that means "an electrical instrument used to monitor underwater sounds" is called a hydrophone. A hydrophone is a kind of microphone that is specifically used to detect and record sound waves underwater.
