Final answer:
The correct completion for the sentence is C) 'are packed', indicating present passive action typically performed on the flowers before they are sent to the market.
Step-by-step explanation:
The correct answer to the sentence 'Then the flowers ____ into boxes and sent to the market.' is option C) are packed. This question requires an understanding of verb tenses and passive voice. In passive voice sentences, the subject is receiving the action of the verb. Since the sentence is describing an action that is habitually performed in the present (flowers are regularly packed and sent to the market), the present tense 'are packed' is the appropriate choice. The sentence does not fit the past tense 'were packing' (option A), the active voice 'pack' (option B), or the perfect tense 'has been packed' (option D).