Bread! Apples! Very small rocks!
Note that since this is used as a security question, you want to be VERY careful about where you give that information out. My standard practice -- except when dealing directly with my own bank or someone else who has a legitimate need for this -- is to refuse to answer that question, or to answer it with some other name or password (and to remember what my answer was so I can give the same answer again if/when they need to confirm my identity.)
If your mother "took" you dad's last name (not all women in all cultures do), then her maiden name is the last name she had when she was growing up...probably her dad's last name (although, again, not all women in all cultures do). ex: Miss Mary Martin marries Mister Matthew Manchester and becomes Mrs. Mary Manchester.
Stop it! Your mother will give you "what for" if she finds out you've been trying to hack her account(s). L
The name she had before she married your father.