Which one of the following options shows the correct labelling of the structure marked as A, B, C & D?
 

In the given figure of flower, the structure marked as A, B, C and D are respectively gynoecium, stamen, ovule and thalamus.
Gynoecium constitutes the inner essential whorl flower comprising carpels. Carpel is the unit of gynoecium. Each carpel contains ovary and stigma. Ovules are produced within ovary. Stamen is the male fertilizing organ of a flower, typically consisting of a pollen-containing anther and a filament. Ovule is the part of the ovary of seed plants that contains the female germ cell and after fertilization becomes the seed. Thalamus is an expanded tip of the pedicel. Components of whorls are concentrically arranged on a thalamus.