The correct option is A.
Let’s hear what fun, interesting things.
To choose the correct placement of a comma in the sentence "let’s hear what fun interesting things happened on your summer vacation," we need to consider the rules of comma usage. In this sentence, we are dealing with a list of adjectives (fun, interesting) that describe a noun (things).
According to the rules of standard English grammar, a comma should be placed between coordinate adjectives. These are adjectives which independently modify the noun and are of equal importance in the sentence.
To determine if adjectives are coordinate, you can try placing 'and' between them or switching their order. If the sentence still makes sense, they are coordinate and should be separated by a comma.
In this case, "fun" and "interesting" are coordinate adjectives. Both adjectives independently describe the noun "things," and the sentence would still make sense if we wrote "interesting and fun things" or "interesting fun things."
Therefore, the correct placement of the comma is after "fun," making the sentence: "Let’s hear what fun, interesting things happened on your summer vacation."