Discuss the three punctuation marks that are used to set off a parenthetical expression and why you would choose each one.

what he said

The answer is:

Commas, Long Dashes - - Brackets ()

Explanation:

A parenthetical expression is that one which adds information but not necessarily needs to be included, thus, we can separate it with one of the punctuation marks mentioned above. I'm going to explain briefly their usage when we have a parenthetical expression:

Use two commas if the parenthetical expression appears in the middle, one before and the other one afterwards. Use a long dash exactly the same as commas, - -this is the least common. Brackets are the most commn for parenthetical expressions, and you need to use them only in the middle, making sure you use both to let the reader know when it started and when it finished.