The correct answer is B. The actor wearing a purple dress with white flowers gave a speech after accepting the award.
Step-by-step explanation:
One of the main functions of noun phrases is to act as the subject or agent in the sentence; additionally, these can be identified because despite including many words they refer to a noun which can be a person, object, animal, place.
In sentence B, the section "The actor wearing a purple dress with white flowers" is a noun phrase because all of these words describe a specific actor (noun) that is the subject of the sentence considering this specific actor is the one that "gave a speech".