Step 1: Recall plastid types. Plastids come in three kinds: leucoplasts which are colourless, chloroplasts which are green, and chromoplasts which are coloured. Step 2: Focus on the pigment. Xanthophyll is a yellow carotenoid pigment, so we need the coloured plastid. Step 3: Identify chromoplast. Chromoplasts store fat-soluble carotenoids like carotene and xanthophyll, giving petals and fruits their bright colours. So the answer is chromoplast. Step 4: Reject amyloplast. Amyloplast is a colourless leucoplast that stores starch, as in potato. Step 5: Reject chloroplast. Chloroplast holds chlorophyll for photosynthesis and appears green, so it is not the xanthophyll store named here. Step 6: Reject aleuroplast and conclude. Aleuroplast stores protein. Therefore the plastid storing xanthophyll is the chromoplast. \[ \boxed{\text{chromoplast}} \]
