The correct answer is C. colourful.
The petals of flowers are usually colourful to attract birds for pollination. Birds are attracted to bright colours, and they will often visit flowers that are brightly coloured in order to eat the nectar or pollen. This helps to pollinate the flowers, which is necessary for them to reproduce.
The other options are incorrect because they do not accurately describe the purpose of the petals of flowers. Option A, “prominent”, is incorrect because the petals of flowers are not necessarily prominent. They can be small and delicate, or they can be large and showy. Option B, “beautiful”, is incorrect because beauty is subjective. Some people may find flowers to be beautiful, while others may not. Option D, “scented”, is incorrect because not all flowers have a scent. Some flowers are fragrant, while others are not.
In conclusion, the correct answer is C. colourful. The petals of flowers are usually colourful to attract birds for pollination.