Umbel is a racemose inflorescence where all stalked flower aggregate on the flat receptacle.
Raceme is a racemose inflorescence having main axis shortened flower born acropetally.
Spadix is a racemose inflorescence having pendulous spike with main axis much flattened.
Spike is a racemose inflorescence having sessile flowers.
The correct option is D. Spike is a racemose inflorescence having sessile flowers. Spike is a racemose inflorescence that bears sessile flowers in an acropetal fashion e.g., chaff flower (Achyranthes), bottle brush (Callistemon), Adhatoda vasica, Amaranthus.