Ответ:

--- Python 3.8.3 x64 ---

import collections  

import re

import string

def ClearString(RawString):

   return re.sub(r' +', ' ', re.sub(rf'[{string.punctuation}]|\n', '', RawString))

def main():

   RawString = input()

   OrigString = RawString

   MostCommonWord = collections.Counter(ClearString(RawString).split(' ')).most_common()[0][0]

   RedactedString = re.sub(rf'{MostCommonWord}', MostCommonWord[::-1], OrigString)

   print(RedactedString)

if __name__ == "__main__":

   main()

Объяснение: