This question tests the proper use of verb tense, specifically the simple past tense. The sentence describes an event that occurred entirely in the past. - **A. is falling:** This is present continuous, which doesn't fit the past context. - **B. has fallen:** This is present perfect tense, implying a connection to the present, which doesn't fit this completed past action. - **C. falls:** This is simple present tense, which is not suitable for the past context. - **D. fell:** This is simple past tense, which is correct because it describes a completed action in the past. This is the correct option. The key to the correct answer is that the sentence describes a singular past occurrence, and therefore, the simple past tense 'fell' is required.