31a Person(s) named in 14 and 29 who will accompany you to the United States now.
Let's continue assuming it is your father's DS-230. The 31a asks "is there any one who will immigrate to US at the time as you so you will travel together" and 31b "is there any one who will immigrate to US at the time as you but will travel later raher than together". So, you're naturally excluded for being a USC.
You mentioned to have applied GC for both parents. So if your mother will travel with your father, place her name on 31a of your father. If she travels separately to join your father later in US, place her name on 31b.
Further, if you had not applied GC for your mother, her name can not be on 31a but 31b.
