The active voice sentence is "Chefs use these machines to mix the ingredients." Choose the option that best expresses this sentence in the passive voice.

Introduction / Context:
This question examines the process of converting a general present tense active sentence into present tense passive voice. The original sentence describes a typical practice in kitchens: chefs use certain machines to mix ingredients. In the passive version, the emphasis should shift to "these machines" while still expressing a general, habitual action.

Given Data / Assumptions:

• Active sentence: Chefs use these machines to mix the ingredients.
• Subject: chefs.
• Object: these machines.
• Verb: use (simple present).
• Purpose clause: to mix the ingredients.

Concept / Approach:
The simple present passive structure is: subject (former object) + is/am/are + past participle. Here, the object "these machines" becomes the subject. Since "machines" is plural, we use "are." The verb "use" changes to the past participle "used." We keep the phrase "to mix the ingredients" to show purpose and add "by chefs" as the agent phrase to indicate who performs the action. This preserves the habitual sense of the original sentence.

Step-by-Step Solution:

Step 1: Identify "these machines" as the object that will become the subject in the passive sentence. Step 2: Note that the verb "use" is in the simple present; in passive for a plural subject, this becomes "are used." Step 3: Keep the infinitive phrase "to mix the ingredients" unchanged at the end of the sentence, because it expresses purpose. Step 4: Add "by chefs" as the agent phrase. The complete passive sentence is: "These machines are used by chefs to mix the ingredients."

Verification / Alternative check:
The passive sentence "These machines are used by chefs to mix the ingredients" accurately reflects the original meaning. It is in simple present, indicates a habitual or general action, and correctly shifts emphasis to "these machines." The grammar is standard and suitable for exam purposes.

Why Other Options Are Wrong:

Option B: "Using of these machines is done by chefs to mix the ingredients." is an overly complex and unnatural way to express the idea; the nominalisation "using of these machines" is unnecessary. Option C: "Chefs mix the ingredients using these machines." remains in active voice and does not perform a voice transformation. Option D: "These machines had been used to mix the ingredients by chefs." changes the tense to past perfect, which is not equivalent to the simple present habitual sense. Option E: "These machines will be used by chefs to mix the ingredients." is simple future passive, indicating a future plan rather than a usual practice.

Common Pitfalls:
Students sometimes unnecessarily use perfect or future tenses when changing to passive, thereby changing the time reference. Others create heavy structures with "is done" or "is being done," which may be grammatically possible but are stylistically poor. When the original sentence is simple and expresses a general truth or habit, the passive version should also be simple and in the same tense.

Final Answer:
The correct passive voice sentence is These machines are used by chefs to mix the ingredients.