The correct option is
B Gynoecium
The whorl of carpel is the inner-most whorl in the flower and it is called the gynoecium. The gynoecium represents the female reproductive organ whorl.
A. The androecium is the whorl of stamens, the male reproductive parts of the flowers.
B. The gynoecium is the whorl of carpels, the female reproductive parts of the flowers.
C. Pistil refers to the fused carpels. In such flowers where the carpel is single, pistil represents single carpel.
D. Gynostegium refers to the structure that is formed by the fusion of stamens, style and stigma. It is seen in Asclepiadaceae.
Hence, the correct answer is 'Gynoecium'