Floral structure hindering natural pollination in Vanilla is

Answer: Option A

The floral structure hindering natural pollination in Vanilla is Rostellum.
The flowers are borne in axillary racemes and each inflorescence consists of 15 - 20 flowers. The flowers are to be artificially pollinated (hand pollination) for the fruit set.
The pollen of the vanilla flower is produced in a mass called pollinia, and is covered by a hood or anther cap. The stigma is protected with a lip known as 'rostellum'. For pollination, the stamen cap is removed by a needle exposing the pollinia. Then the flap-like rostellum is pushed up and the pollinia are brought into contact with the stigma.