。
She wants to know who sent her the flowers.
Literal
She [topic-は] who [subject-が] flowers [object-を] sent-for-her [explanatory question-のか] [wants to know-知りたがっている].
Multiple grammar pieces stacked together. 贈ってくれた is 贈る ('to give as a gift, to present') plus the benefactive ~てくれる, which marks the action as done for the speaker or in-group's benefit — so the flowers came to her, not from her. The embedded question 誰が花を贈ってくれたのか is the object of 知りたがっている, with ~のか treating the question reflectively ('the question of who...'). Finally ~たがっている is the third-person desiderative: ~たい becomes ~たがる when describing someone else's wants (you can't directly assert another person's interior feelings in Japanese), and ~ている marks the wanting as ongoing. The whole construction is a fine demonstration of how Japanese refracts emotion through grammatical conventions.