about what
¿Sobre qué estaban discutiendo?
What were they arguing about?
"Acabo de hablar con Tom.""¿Sobre qué?""Sobre lo que hizo Mary."
“I just spoke to Tom.” “What about?” “About what Mary did.”
Cuéntame lo que me ibas a contar ¿Sobre qué?
Tell me what you were going to tell me. What about?
— El espíritu de la colmena (1973)
¿Sobre qué te gustaría conocer más?
What would you like to know more about?
What are you mumbling about?