Correct Answer:
B. was cooking
The correct answer is B: was cooking. This sentence uses the past continuous tense, which is perfect for describing an ongoing action in the past that was interrupted by another past action. The structure of the past continuous is "was/were + verb-ing." In this context, "he was cooking dinner" describes an activity that was in progress when the speaker "arrived," which is a specific, shorter past action that interrupted or occurred during the longer cooking activity.
Let's examine why the other options are unsuitable:
- A: cooks (Simple present) is used for habitual actions or general truths, not for a specific past event.
- C: cooked (Simple past) would imply that he finished cooking dinner at the moment I arrived, or that he cooked dinner and then I arrived, which doesn't capture the sense of an ongoing action.
- D: has cooked (Present perfect) connects a past action to the present, suggesting the cooking is either recently finished or has an effect on the present, which doesn't fit the narrative of an interruption by a past event ("when I arrived").
Thus, "was cooking" accurately portrays the continuous nature of the action at the moment of arrival.