In this active and passive voice question, the sentence The candidates are writing an essay is given in the active voice. Choose the option that correctly expresses the same idea in the passive voice.

Introduction / Context:
This question tests the ability to change a present continuous sentence from the active voice to the passive voice. The original sentence is The candidates are writing an essay, where the subject candidates performs the action of writing. In the passive voice, the focus shifts to the object an essay, while preserving the tense and aspect.

Given Data / Assumptions:

• Active sentence: The candidates are writing an essay.
• Subject: the candidates.
• Verb phrase: are writing, present continuous.
• Object: an essay.
• The passive form must keep the same time reference and continuous aspect.

Concept / Approach:
To convert a present continuous active sentence to passive, the structure is subject plus am or is or are plus being plus past participle. The object moves to subject position. Thus, an essay becomes the new subject. The auxiliary are becomes is, because essay is singular. Then we add being and the past participle written. Finally, we may include the original subject in a by phrase, resulting in An essay is being written by the candidates.

Step-by-Step Solution:
Step 1: Identify the object an essay, which will become the subject in the passive sentence. Step 2: Note the tense and aspect: are writing is present continuous. Step 3: Form the passive auxiliary for a singular subject: is being. Step 4: Use the past participle of write, which is written. Step 5: Add the agent with by, giving An essay is being written by the candidates.

Verification / Alternative check:
We can check by reversing the passive back to active. From An essay is being written by the candidates, we obtain The candidates are writing an essay, which matches the original sentence exactly in meaning and tense. The presence of is being written confirms the correct continuous passive form. This shows that option b is the correct transformation.

Why Other Options Are Wrong:
Option a, An essay was wrote by the candidates, has two problems: the tense changes to past simple and wrote is not a correct past participle. Option c, An essay has had been written by the candidates, is an incorrect and overcomplicated tense that does not correspond to the present continuous. Option d, An essay had been written by the candidates, is past perfect passive, again not matching the original time frame. Option e, An essay is written by the candidates every day, expresses a general habitual action rather than an action in progress right now, so it changes the meaning.

Common Pitfalls:
Students sometimes forget to include being in continuous passive forms and write is written instead of is being written. Another common mistake is to change the tense unintentionally, for example shifting from present continuous to past or present simple. A clear mental formula for present continuous passive is is or am or are plus being plus past participle, which should be applied consistently.

Final Answer:
The correct passive voice form of The candidates are writing an essay is An essay is being written by the candidates, so option b is correct.