Question

Select the noun phrase to complete the sentence.
The Flowers ______ bloom every year in april
(pictures are my choices)

Answer 1

Well, the only noun phrase I see that makes sense is: "dark purple irises."

The first one isn't a noun phrase, and the third one would have needed revising for it to make any sense. 

Answer 2

There is only one noun phrase in the picture. That is, your answer would be "dark purple irises". "Slowly but surely" is not a noun phrase, but an adverbial phrase and "with the purple petals" is a prepositional phrase.

In this way, the complete sentence would be as follows: The flowers, dark purple irises, bloom every year in April. This NP is an apposition as the two nouns refer to the same thing (flowers/irises) and are placed immediatly after the other with no conjunction between them.