Flowers which have a single ovule in the ovary and are packed into an inflorescence are usually pollinated by:

The correct option is D. Wind. Wind-pollinated flowers have a single ovule in the ovary, and the flowers are packed into an inflorescence. Pollen is lightweight and non-sticky, which leads to easy dispersal of the pollen from the anther to the stigma of the flower by wind. For example, corn cob, where the pollens are packed in an inflorescence. So, the correct answer is option D.