Select the option that expresses the given sentence in passive voice.

Mary was cooking the meal.

The meal was being cooked by Mary.

The correct passive form of the given sentence Mary was cooking the meal is The meal was being cooked by Mary.

The active voice of the sentence is Mary was cooking the meal. This sentence is in the past progressive tense, which is used to describe an action that was happening in the past but was not finished. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "meal" and the main verb is "cook." The past participle of "cook" is "cooked." So, the passive form of the sentence is The meal was being cooked by Mary.

The other options are incorrect because:

• The meal were being cooked by Mary. The subject of the sentence is singular, so the verb "were" should be "was."
• The meal was already being cooked by Mary. This sentence implies that the meal was already finished being cooked, which is not the case.
• Mary was about to cook the meal. This sentence is in the future progressive tense, which is not appropriate for describing an action that was happening in the past.
• The meal was being cooked. This sentence does not specify who was cooking the meal, so it is not in the passive voice.