The correct option is D Stamen
The site of formation of pollen grains is in the stamen because stamen is the central part of the male reproductive part in higher plants and it consists of filament tubes like filament and anther. Inside the anther, diploid cells undergo meiosis and produce microspores or haploid male spores or pollen grains. Whereas pistil is the female reproductive part and receptacles and petals are the nonreproductive part. So, the correct answer is option stamen.