Any plant organ can be a sink while it is developing. Leaves can storage organs are sources when they send photosynthate or reserves to other parts of the plant, and storage organs and roots are generally sinks while taking up photosynthate from the phloem. Fruits and seeds are usually just sinks while they develop.
Work Step by Step
Just review the concept and use your imagination to think about a plant and what parts send sugars out (leaves and sometimes storage organs) versus which ones take up that sugar.