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C is the clearest and easiest to read. The commas not only set off the interrupter to highlight it but give the reader a moment to pause and note the two items being discussed (least and most).
Answer
#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/24961 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/24961 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/