SOLUTION: Which of the following is the negation of: We're all gonna be a lot thinner.

To get the negation, just stick in a NOT to modify the verb:

"We're NOT all gonna be a lot thinner."   <-- ANSWER

It's OK to reword to make it sound better as long as you don't change the
meaning.  For instance, you could reword this as

"Not all of us are gonna be a lot thinner."

Edwin