It's mother's birth... To explain:Hey can I ask you another question. this might sound stupid lol but on the passport application one of the question is "mother's surname at birth" under personal information. Do you know if they are talking about her birth or mine? It doesn't specify whose birth they asking this information for!!
It's your mother's maiden name before she got married... If she has changed her name after marriage(s), it's the one that's on her birth certificate