。
Where did she buy the book?
Literal
She [topic-は] where [at-で] book [object-を] bought [question-か]?
で marks the location of an action — where the buying happened. Distinct from the destination に (where one goes) and the existential に (where something is). Three different roles, three different particles, all important to keep straight: 本屋で本を買った (bought at a bookstore), 本屋に行った (went to the bookstore), 本屋に本がある (there are books in the bookstore).