B. Everybody's Billy is delighted that everybody's invitations arrived on time.
The general rule to indicate ownership is to put an apostrophe + s at the end of a word. The exceptions are if the word is plural and uses an s or es to indicate that, or if it is a proper noun ending in s. "Everybody" is neither, so B is correct.
The pronouns everybody,somebody and nobody are always singular That's why the apostrophe goes before the noun as they always do in singular nouns in the possessive case So, the correct answer is 'b'
