We need to learn all uses of "su".
If a sentence is about a boy, and includes the word "su" then "su" would mean,"his". Or if the sentence was about a girl, then "su" would mean "her."
However, in most cases, there is no certain CORRECT usage apart from whoever has been priorly named.
The informal (tú) your would be tu (without the accent mark), but the formal (Usted) your is su.
His/her and their is also su. This means you need context to know what is meant, and since there is no context, the following translations should be accepted (if they're not, use the report button):
Your (formal) grandfather is from Mexico.
Their grandfather is from Mexico.
His grandfather is from Mexico.
Her grandfather is from Mexico.