Here are some possible interpretations, depending on the context:
* Who is Sophie's? This could mean who owns or belongs to Sophie.
* Who is from Sophie? This could refer to someone who originates from a place or person named Sophie.
* Who is of Sophie? This is a less common expression but could refer to someone who is related to or associated with Sophie in some way.
To understand the meaning accurately, more context is needed. For example, where did you see this phrase? What was the surrounding conversation?