Add a prepositional phrase to clarify either to whom an item(s) belongs.
Cual es su problemas? (ella)
Thank you for using the Jiskha Homework Help Forum. Don't be confused! A prepositional phrase is often used to clarify "su" because it could mean "hers, his, your-formal-singular, their (masculine or feminine), you-all-formal-plural.
Here are the clarification phrases:
de él, de ella, de usted, de ellos, de ellas, de ustedes.
¿Cuál es su problema? (NOTE: It is impossible to have "problemas" plural if you begin with "Which IS....." The answer, in this case, would be "Es su problema de ella." or simply "Es de ella."
P.S. The other preposition phrases are:
mi problema = de mí
tu problema = de ti
nuestro problema = de nosotros
vuestro problema = de vosotros
I think you made a typo error in your question.
"su" (singular) does not go go with problemas (plural). It would have to be "sus problemas" or "su problem".
In any case, I agree with SraJMcgin's anwer
Make that "su problema", not "su problem". Sorry