The correct option is C Carpels
The fourth whorl of the flower is of carpels. The pistils of a flower are considered to be composed of carpels. A pistil may consist of one carpel, with its ovary, style and stigma or several carpels may be joined together with a single ovary, the whole unit called as a pistil. The gynoecium may consist of one or more uni-carpellate (with one carpel) pistils, or of one multi-carpellate pistil
So, the correct answer is 'Carpels'