Final answer:
The sentence should be completed in a way that fits into the context provided, relating to business or formal writing guidelines. A phrase like 'without exception' indicates that all plant buyers are included in the requirement to pay the penalty.
Step-by-step explanation:
To effectively complete the sentence 'An important aspect of this plan is that all plant buyers, __________, will be required to pay the penalty', it's necessary to fill in the blank with information that is contextually relevant and grammatically correct. Given the additional information provided, the sentence seems to relate to business or formal writing guidelines, discussing aspects such as the Prevention Pollution Act and effective communication.
A proper completion might focus on the inclusivity or responsibilities of the plant buyers within the context of the Prevention Pollution Act. Without more explicit context, one could complete the sentence with 'without exception', implying that the rule applies uniformly to all plant buyers. Therefore, the completed sentence would read: 'An important aspect of this plan is that all plant buyers, without exception, will be required to pay the penalty.'