Final answer:
The bee sticks its proboscis into the stigma of the flower and transfers pollen.
Step-by-step explanation:
The part of the flower that the bee sticks its proboscis into and transfers pollen to is the stigma. The stigma is part of the pistil, the female reproductive structure of a flower.
It is responsible for receiving and guiding the pollen grains to the ovary, which contains the female gametes.