The sentence "The breakfast will have been finished by eight" is given in the passive voice. Choose the option that correctly expresses the same meaning in the active voice.

Introduction / Context:
This item tests your ability to change a future perfect passive sentence into its corresponding active form. The original sentence, The breakfast will have been finished by eight, states that the action of finishing breakfast will be completed before or by a certain future time. In the active sentence the focus moves from breakfast, the thing acted upon, to the implied doers of the action, they. The correct option must therefore use a future perfect active construction that preserves the sense of completion by eight o'clock.

Given Data / Assumptions:
The sentence provides the following elements.

• Original passive: The breakfast will have been finished by eight.
• Tense and aspect: future perfect passive (will have been finished).
• Passive subject: the breakfast.
• Agent: implied they, not explicitly stated in the sentence.
• Time reference: by eight, showing a point in the future before which the action will be complete.

Concept / Approach:
The future perfect passive form follows the pattern will have been + past participle. To convert this to active voice, we keep the future perfect structure but remove been and make the agent the new subject. The pattern becomes subject + will have + past participle. Here the natural active subject is they, representing the people who will eat or prepare the breakfast. The verb finished remains as a past participle after have. The phrase by eight is essential because it signals that the action will be completed not at a single moment exactly at eight but at some time before or up to eight o'clock. Thus, the correct active sentence must be They will have finished the breakfast by eight.

Step-by-Step Solution:

Verification / Alternative check:
To verify, take the active sentence They will have finished the breakfast by eight and convert it back into passive voice. The object the breakfast becomes the subject, the future perfect active verb will have finished becomes the future perfect passive will have been finished, and the agent they is removed or placed in an optional by phrase. The resulting sentence is The breakfast will have been finished by eight, which matches the original. This confirms that the active option we have chosen is correct and preserves the meaning and time reference.

Why Other Options Are Wrong:
Option A, They will finish the breakfast by eight, uses simple future instead of future perfect. Although close in meaning, it does not express the same emphasis on completion before a certain time. Option B, They will finish the breakfast at eight, changes both the tense and the preposition and suggests that completion occurs exactly at eight o'clock. Option D, They will be finishing the breakfast at eight, is future continuous and focuses on an ongoing action at that time, not a completed one. Option E, They have finished the breakfast by eight, uses present perfect and completely changes the time frame. Only option C correctly uses future perfect and preserves the by eight time expression.

Common Pitfalls:
Many learners do not distinguish clearly between at and by when talking about time, even though the meanings differ. In perfect tenses, by is important because it indicates a deadline or upper limit for completion. Another frequent error is to ignore the presence of have been in a passive verb phrase and to choose a simple future form instead. When dealing with tenses and voice, always break complex verb phrases into components, identify the tense and aspect, and then apply the corresponding active or passive pattern. This method prevents confusion between simple future, future continuous, and future perfect.

Final Answer:
The correct active voice sentence is: They will have finished the breakfast by eight.