What is the difference between the following?
[ I am on a bus] Could you stop by/at/on/in 23rd and Pine?
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"Stop at" is a definitive stop, with the emphasis on a particular location; this is the most appropriate usage for the bus making a stop.
When used as a verb "stop by" connotes a brief pauses, and usually implies continued motion after the pause. "By" could also be used as a synonym of "near", as in "Stop by (near) the intersection", with a meaning more similar to "stop at". The difference here is subtle and usually requires more context to distinguish.
"Stop on" and "stop in" wouldn't apply here. "Stop on" might refer to stopping in a bounded or contained region (a chess piece might "stop on" a square), "stop in" is generally used for a person visiting another person, often in a building or other closed structure (ex: "Stop in after work, I'd love to see you.")