Answer:
D. The flowers in the window box need watering.
Step-by-step explanation:
The subject verb agreement suggests how the sentences are framed and if they are correct or not. Subject - verb agreement is the correct use of the verbs in accordance to the subjects in the sentence.
Option A is wrong as the subject 'flower' is singular and the verb 'need' should be 'needs'.
Option B is wrong as the subject is plural which makes the verb 'needs' invalid. It should be 'need'. Same case with option C.
Assuming the whole sentence is "the flowers in the window box need watering", the correct answer will be option D -
The flowers (subject) in the window box need (verb) watering.