。
Do you know what kind of flowers to buy for Mother's Day?
Literal
Mother's day [on-には] what kind-of flowers [obj-を] if-bought good [question-か] know (polite)?
The chain ~たらよいか is a fixed 'what one should (do)' frame: 買ったらよい = 'it would be good if one buys,' か closes the embedded question, and the whole thing is the object of 知っている. The particle には on 母の日 (に + は) topicalizes the time within a contrast ('as for Mother's Day, specifically...'). どんな asks about the kind or category of flowers.