Which sentence or sentences use parentheses to set off an incidental remark? If you don't start studying more (I mean this), you
will fail sixth grade. I wore a rose corsage (I prefer roses to carnations) to the banquet. We need the following items: (1) food, (2) shelter, and (3) clothing. I left for college when I was 18 (that was also the age at which I was married).
I believe it is the final sentence: I left for college when I was 18 (that was also the age at which I was married). This is because the parentheses also give information about another incident that took place at the time - his/her marriage.
An outline provides the key words or the important details in a text which basically helps the readers focus and analyze more what the text is trying to convey.