Answer:
Boast "of"
Explanation:
What preposition is used with boast?
For the most part, the two prepositions can be used interchangeably. According to The Free Dictonary, "boast of" and "boast about" are practically the same idiom. For example, both of these are valid, The chef boasts of his restaurant having the most authentic Italian food in all of New York City.
