Which of the following passive interrogative sentences correctly expresses the active question "Who burned the food?"?

Introduction / Context:
This question focuses on transforming an active voice question into the passive voice while preserving tense, reference, and correct pronoun case. The active sentence is Who burned the food? Your task is to select the passive question that matches this meaning and uses proper English structure.

Given Data / Assumptions:

• Active question: Who burned the food?
• Subject (doer) in active: an unknown person (who).
• Object in active: the food.
• Tense: simple past (burned).
• Options are different passive forms starting with By who or By whom, and with variations in this/that/the and in tense.

Concept / Approach:
To convert a who question in active voice to passive voice, we move the object to the subject position, keep the tense consistent in the passive auxiliary, and use by whom to refer to the unknown agent. The correct passive question pattern is: By whom + was + object + past participle? Thus, Who burned the food? becomes By whom was the food burned? The demonstrative this or that would change the reference, so we need the food and not this food or that food.

Step-by-Step Solution:
Step 1: Identify tense and object in the active sentence: simple past burned, object the food.Step 2: Form the passive subject from the object: the food.Step 3: Use the simple past passive auxiliary: was burned.Step 4: Introduce the unknown agent with by whom at the beginning of the question, as passive interrogatives often do.Step 5: Combine to form: By whom was the food burned?, which exactly matches option C.

Verification / Alternative check:
Check each option carefully. Option A uses By who instead of By whom, which is incorrect in formal grammar because who here is the object of by. Option B uses that food instead of the food, which slightly changes the reference and is not an exact transformation of the original sentence. Option D uses is instead of was, changing the tense from past to present, and this food instead of the food. Only option C preserves the tense, the article, and uses the correct object pronoun whom.

Why Other Options Are Wrong:
By who was this food burned? (Option A) contains two problems: who instead of whom after a preposition, and this food instead of the food. By whom was that food burned? (Option B) gets the pronoun right but changes the determiner, which is typically considered a deviation in strict transformations. By whom is this food burned? (Option D) shifts from past to present continuous or present simple passive, altering the time reference.

Common Pitfalls:
Many learners struggle with whom vs who and may think By who is acceptable. In formal written English and in exams, when the pronoun follows a preposition like by, whom is the preferred and grammatically correct choice. Another common error is to forget tense consistency and accidentally change was burned to is burned. Always carry over the tense while shifting from active to passive.

Final Answer:
The correct passive interrogative form of Who burned the food? is By whom was the food burned?, so option C is correct.